Control 'gvItemUpdate' of type 'GridView' must be placed inside a form tag with runat=server.

Control 'gvItemUpdate' of type 'GridView' must be placed inside a form tag with runat=server
The above exception occurs when one tries to export a GridView control to Word, Excel, PDF, CSV or any other formats.

Reason:
.NET compiler thinks that the control is not added to the form and is rendered.

Solution:
Tell compiler that the control is rendered explicitly by overriding the VerifyRenderingInServerForm event.
Something like:

public override void VerifyRenderingInServerForm(Control control)
{
    /* Verifies that the control is rendered */
}

This Error/issue reported & resolution suggested by MS here: RenderControl doesn't work for GridView[^]

may be your gridview is misplaced please keep in ur code runat="server" or check once your code inside tag r not