There doesn't seem to be any strange issue like "the last area in the Area Manager is missing from area choosers", at least not on my boards.
So let's take a moment to think about the config.
1) In order to move content to an area, you must also have permission to create that content in the new area.
2) Even if you are an admin with all permissions, the area must also be configured to accept the content you are moving. It is possible that the Sandbox area config does not allow:
- The page is a chapter in a book, and the new area does not accept chapters.
- The existing content-type, or additional "treat this page as", for the page you are moving.
- One of the additional "treat this page as" is globally disabled (so the content is no longer allowed anywhere).
- The existing prefix of the page you are moving.
- The existing language of the page you are moving.