In your folder ...\Packages\Community\pmdg-aircraft-738-liveries there should be a layoutgenerator.exe
Can you test if it works if you drag the exe file onto the layout.json as in the picture?
https://drive.google.com/file/d/1qtlglpzeO1ykxkUubCxjNfOK4rsvQy41/view?usp=drive_link
Anyway that behavior is not normal. When I install a livery it works automatically in FS24...