How to fix ” The Controls collection cannot be modified because the control contains code blocks (i.e. ). “

lets say you have this in your head tag  in a master page

<script type=”text/javascript” src='<%=ResolveClientUrl(“~/js/js.js”) %>’></script>

replace the above with

<script type=”text/javascript” src='<%#ResolveClientUrl(“~/js/js.js”) %>’></script>

and then add

VB.NET

Protected Overloads Overrides Sub OnLoad(ByVal e As EventArgs)
MyBase.OnLoad(e)
Page.Header.DataBind()
End Sub

C#

protected override void OnLoad(EventArgs e)
{
base.OnLoad(e);
Page.Header.DataBind();
}