The page for which you are getting this error must have defined a master page for itself in it's Page attributes:
MasterPageFile="~/MasterPage.master"
Once you do it, your aspx should have a Content page such that master page can use it while rendering. Your page design here does not have any content panel in it.
Two options:
1. Remove page attribute for master page. => remove
MasterPageFile="~/MasterPage.master"
OR
2. Add Content control in your Declaration_Form.aspx. Add you page design in that content. (If needed, you can refer the design of content page looking at other pages.)
Read all about them here:
MSDN: ASP.NET Master Pages[
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