Error message it pretty clear,you can not use contentplaceholder in content pages. If your masterpage contains
<asp:ContentPlaceHolder id="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
Then to place content in this contentplaceholder,in content page your code should be like
<asp:Content ID="Content1" runat="server" ContentPlaceHolderID="ContentPlaceHolder1">
</asp:Content>
Regards....