This is a known bug, some work-arounds are:
1. Delete the localthumbcache.package from the Sims 4 folder. This helps clear up some game glitches.
2. End the other romantic relationships with other sims - if any.
3. Remove relationship then build their relationship back up to where it was.
4. Dismiss the employed Sim and try again.
This glitch or bug may be from trying to marry service Sims, or a Sim that had a previous relationship. Read more about it in the thread linked below:
Re: I Can Not Get Married
https://answers.ea.com/t5/Bug-Reports/WORKAROUND-Can-t-get-married-because-Sim-works-for-me/m-p/6443706
