It’s possible gibbousmoon is trying to add a synonym for a vocab item that hasn’t been learned yet? In advance? It won’t let you do that. You have to do it their way at least one time (unless you add the script for adding synonyms during lessons, if it’s working, which I have no idea if it is or not, just blathering now).
Oh, just fully read this for the first time. Not sure why you got the impression that you can only do synonyms at a specific time. You can add them any time you like after a lesson.