Product and version
FileMaker Pro Advanced 15.0.1
OS and version
OS X 10.10.5
iMac Retina 5K 27'' Late 2014
In Manage Layouts dialog, if the most front Document window have custom menu installed, New Layout button is greyed out and you cannot push it.
How to replicate
Create a new database and an empty custom menu (the default).
Open the Manage Layout dialog
On your document window, install the empty custom menus
Going back to the manage layout dialog, you cannot add a new layout
Keep a window in Layout mode or with standard menus installed and keep it in the most front when calling the manage dialog.
Interestingly, i was running into this issue for the first time with FileMaker Pro 15 some days ago, but i now can also replicate the issue with FileMaker 14. Strange, since i work with FileMaker Pro 14 all the days, full time since one year.