Hello,
Below code is work for me.
Try to use below code in aspx page,
<asp:linkbutton runat="server" id="lnkDownload" commandname="DELETE" ondatabinding="lnkDownload_DataBinding" xmlns:asp="#unknown"></asp:linkbutton>
and use below code in aspx.vb
Protected Sub lnkDownload_DataBinding(sender As Object, e As EventArgs)
Dim lnkDownload As LinkButton = DirectCast(sender, LinkButton)
ToolkitScriptManager1.RegisterPostBackControl(lnkDownload)
End Sub