Just a general note: try to avoid the "Other unit files"
Instead: Create a "new package".
Save or copy the form into the package dir, and add it to the package. Or if you want to create a new form, choose add in the package, and choose "Form".
That way the IDE should manage all dependencies correctly. (Of course if you copied , then you may still have to tell the package about dependencies first.)
You can put all your forms into one package.