Hi nauna,

Check this sample. now take its reference and correct your code.

I have created this sample using this link.

Download multiple files as Zip Archive File in ASP.Net using C# and VB.Net

HTML

<asp:Button ID="btnDownload" Text="Download All File" runat="server" OnClick="OnDownload" />

Namespaces

C#

using Ionic.Zip;

VB.Net

Imports Ionic.Zip

Code

C#

protected void OnDownload(object sender, EventArgs e)
{
    using (ZipFile zip = new ZipFile())
    {
        zip.AlternateEncodingUsage = ZipOption.AsNecessary;
        zip.AddDirectoryByName("Files");
        string lbl3dfiles = Server.MapPath("~/Files/Test.doc");
        string lblstl = Server.MapPath("~/Files/Test.pdf");
        string lblgcode = Server.MapPath("~/Files/Test.txt");
        string lblprintcoder = Server.MapPath("~/Files/Test.xls");
        if (lbl3dfiles != string.Empty)
        {
            zip.AddFile(lbl3dfiles, "Files");
        }
        if (lblstl != string.Empty)
        {
            zip.AddFile(lblstl, "Files");
        }
        if (lblgcode != string.Empty)
        {
            zip.AddFile(lblgcode, "Files");
        }
        if (lblprintcoder != string.Empty)
	    {
		    zip.AddFile(lblprintcoder,"Files");
	    }
        Response.Clear();
        Response.BufferOutput = false;
        string zipName = String.Format("Zip_{0}.zip", DateTime.Now.ToString("dd-MMM-yyyy-HHmmss"));
        Response.ContentType = "application/zip";
        Response.AddHeader("content-disposition", "attachment; filename=" + zipName);
        zip.Save(Response.OutputStream);
        Response.End();
    }
}

VB.Net

Protected Sub OnDownload(sender As Object, e As System.EventArgs) Handles btnDownload.Click
    Using zip As ZipFile = New ZipFile()
        zip.AlternateEncodingUsage = ZipOption.AsNecessary
        zip.AddDirectoryByName("Files")
        Dim lbl3dfiles As String = Server.MapPath("~/Files/Test.doc")
        Dim lblstl As String = Server.MapPath("~/Files/Test.pdf")
        Dim lblgcode As String = Server.MapPath("~/Files/Test.txt")
        Dim lblprintcoder As String = Server.MapPath("~/Files/Test.xls")
        If lbl3dfiles <> String.Empty Then
            zip.AddFile(lbl3dfiles, "Files")
        End If
        If lblstl <> String.Empty Then
            zip.AddFile(lblstl, "Files")
        End If
        If lblgcode <> String.Empty Then
            zip.AddFile(lblgcode, "Files")
        End If
        If lblprintcoder <> String.Empty Then
            zip.AddFile(lblprintcoder, "Files")
        End If
        Response.Clear()
        Response.BufferOutput = False
        Dim zipName As String = String.Format("Zip_{0}.zip", DateTime.Now.ToString("dd-MMM-yyyy-HHmmss"))
        Response.ContentType = "application/zip"
        Response.AddHeader("content-disposition", "attachment; filename=" & zipName)
        zip.Save(Response.OutputStream)
        Response.[End]()
    End Using
End Sub

Screenshot