This is expected behavior when filling a copy. If you need to delete a page from the PDF, you’ll need to do that while in normal edit mode for the template. The workflow to fill a copy doesn’t include deleting or adding pages since it’s supposed to be an identical copy of the original file and just with new field values. You can still add new annotations to the copy by using the Tool Bar, but you can’t use the Page Controls actions to delete or rotate or add a page, If you don’t own this template, you can make a copy of it first and then use Page Controls or the File > Actions menu to append a page. You can learn more about appending a page at the link below. The link will even take you to the section of the article for appending files:
What PDF edits can I make to a document?