Open Manage Layouts has been a script step since Filemaker Pro 11, so this is either a new bug or something that has gone unreported for a long time.
Product and version FileMaker Pro Advanced 17.02
OS and version: MacOS 10.13.6
Description When Manage Layouts opens as a modal window, the buttons at the bottom for NEW, EDIT, DELETE, DUPLICATE are dimmed not allowing the user to create a new layout or edit an existing one. However, if the user uses a cursor to select the parent FileMaker layout window, and then returns to Manage Layouts, everything is functional.
The Condition is illustrated here:
This is the best workaround I can think of. Add a script step after Open Manage Layouts which selects the other window by its name. Then, the user will have to click on Manage Layouts on their own. If you add Open Manage Layouts after Select Window, it just repeats the same bug.