Thank you for your post.
If you do not have access to View -> Layout Mode, then you will be unable to create a layout. Therefore, add the View menu to your blank custom menu, and be sure Layout Mode has not been removed. Then, you will be able to add a new layout.
However, I am perplexed by your answer : did you see the workaround i mentioned ?
Why the menu set of the most front window should affect the dialog box Manage Layout ?
Sounds like the design is as old as the time where FileMaker was limited to one window by file...
Not so important but i saw an inconsistency and it was the reason of my report.
If you select another window that has a different menu (standard or custom) with View -> Layout Mode, then the "New" layout option will be available.
yes i know -> see my previous posts and in particular my workaround.
Maybe it is my english and i am unable to make me understand...
Have a nice day !