ASP.Net System.Data.SqlClient.SqlException: Must declare the scalar variable

System.Data.SqlClient.SqlException: - Must declare the scalar variable.

public string testquery(string Transaction, List<string> columns, List<object> data, string tableName)
{  
    var strCol = string.Join(",", columns);
   
    var strParam = string.Join(",", columns.Select(r => "@" + r));
   
    var sql = "INSERT INTO [" + tableName + "] (" + strCol + ") VALUES (" + strParam + ")";
 
    SqlConnection con = new SqlConnection(_conString);
    if (con.State == ConnectionState.Closed)
    {
        con.Open();
    }
    SqlCommand command = new SqlCommand();
    command.Connection = con;
    command.CommandText = sql;
 
    for (int i = 0; i < columns.Count; i++)
    {
        command.Parameters.AddWithValue(strParam[i].ToString(), data[i]);
    }
 
    command.CommandType = CommandType.Text;
    if (command.ExecuteNonQuery() > 0)
    {
        con.Close();
        return "SUCCESS";
    }
    else
    {
        con.Close();
        return "Fail";
    }
}

The above function is called in page load.

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        List<string> columns = new List<string>() { "u_ip" };
        List<object> data = new List<object>() { 110654965406 };
        String tableName = "Newuser";
        testquery(null, columns, data, tableName);
    }
}