Fresher says:
 $(function () {
    $("[id*=btnAgregar]").click(function () {
        var checked_checkboxes = $("[id*=projectchk] input:checked");
        var message = "";
        checked_checkboxes.each(function () {
            
            var text = $(this).closest("td").find("label").html();
            message += "Text: " + text;
            message += "\n";
        });
        alert(message);
        return false;
    });
});
Replace the above with the below.
$(function () {
    $("[id*=btnAgregar]").click(function () {
        var checked_checkboxes = $("[id*=projectchk] input:checked");
        var message = "";
        checked_checkboxes.each(function () {
            var text = $(this).closest("span").find("label").html();
            message += "Text: " + text;
            message += "\n";
        });
        alert(message);
        return false;
    });
});
Or
$("[id*=btnAgregar]").click(function () {
    var checked_checkboxes = $("[id*=projectchk] input:checked");
    var message = "";
    checked_checkboxes.each(function () {
        var text = $(this).parent().find("label").html();
        message += "Text: " + text;
        message += "\n";
    });
    alert(message);
    return false;
});
Both will work in your case.
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
    <script type="text/javascript">
        $(function () {
            $("[id*=btnAgregar]").click(function () {
                var checked_checkboxes = $("[id*=projectchk] input:checked");
                var message = "";
                checked_checkboxes.each(function () {
                    var text = $(this).closest("span").find("label").html();
                    message += "Text: " + text;
                    message += "\n";
                });
                alert(message);
                return false;
            });
        });
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <table cellspacing="0" border="0" id="ctl00_cphGlobalContent_FormView1" style="border-collapse: collapse;">
            <tbody>
                <tr>
                    <td colspan="2">
                        <div class="form-check form-check-inline" id="projectchk">
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl01_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl01$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl01_CategoryID">170449</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl02_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl02$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl02_CategoryID">170450</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl03_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl03$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl03_CategoryID">170451</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl04_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl04$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl04_CategoryID">170452</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl05_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl05$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl05_CategoryID">170453</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl06_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl06$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl06_CategoryID">170454</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl07_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl07$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl07_CategoryID">170455</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl08_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl08$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl08_CategoryID">170456</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl09_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl09$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl09_CategoryID">170457</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl10_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl10$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl10_CategoryID">170458</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl11_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl11$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl11_CategoryID">170459</label></span>
                            <span class="form-check-input">
                                <input id="ctl00_cphGlobalContent_FormView1_Repeater1_ctl12_CategoryID" type="checkbox"
                                    name="ctl00$cphGlobalContent$FormView1$Repeater1$ctl12$CategoryID"><label for="ctl00_cphGlobalContent_FormView1_Repeater1_ctl12_CategoryID">170460</label></span>
                        </div>
                    </td>
                </tr>
            </tbody>
        </table>
        <br />
        <input type="submit" name="btnAgregar" value="Agregar" id="btnAgregar" />
    </div>
    </form>
</body>
</html>
 
Demo