Hi!
There are a couple of different proposal bugs floating around right now in Sims 4. If you're not seeing a propose option at all, you've probably got the one where if Sims were promised as teens they can't get engaged for some reason as Young Adults. To fix this, have one ask the other "to just be friends," (under friendly interactions) and then build their romantic relationship back up from scratch. One they are girlfriends again and the relationship is high enough, you should be able to propose.
If you've got the other bug, where the propose option appears, but is greyed out, make sure neither of them has any other girlfriends or boyfriends, and if they do, break up with them. That should solve the issue.
I hope that helps!