Performance:
Of course, it may be that another control must be placed in the form.
Cause:
In normal cases, you can add a form directly, but if you use the master page, the real cause of this error is that the contentplaceholder in the master page is mistakenly placed outside <form>:
<Form ID = "form1" runat = "server">
<Asp: linkbutton id = "PB" runat = "server"> <B> P </B> roduct backlog </ASP: linkbutton>
<Asp: linkbutton id = "SB" runat = "server"> <B> S </B> Print backlog </ASP: linkbutton>
</Form>
<Asp: contentplaceholder id = "contentplaceholder1" runat = "server">
</ASP: contentplaceholder>
<Asp: contentplaceholder id = "contentplaceholder2" runat = "server">
</ASP: contentplaceholder>
Changed:
<Form ID = "form1" runat = "server">
<Asp: linkbutton id = "PB" runat = "server"> <B> P </B> roduct backlog </ASP: linkbutton>
<Asp: linkbutton id = "SB" runat = "server"> <B> S </B> Print backlog </ASP: linkbutton>
<Asp: contentplaceholder id = "contentplaceholder1" runat = "server">
</ASP: contentplaceholder>
<Asp: contentplaceholder id = "contentplaceholder2" runat = "server">
</ASP: contentplaceholder>
</Form>
Everything works.