So I've been fighting this for months.
And normally it only takes me 5 mins to get married with cheats.
But the option to date does not come up!
I wiped my game cache clean.
Reinstalled patches.
Created new names.
NOTHING still cannot date
I can become romantic interest but that's it
Does the other sim have a steady relationship, or has the sim been married and the spouse died? Sometimes the game won't recognize widowed sims as single and lists their relationship with the ghost as engaged, and thus unavailable to commit to anyone else. But it won't do the "ask to break up with" unless the ghost is spawned in the world.
If you get the "ask to break up with..." thing use that, and it should clear it up.
If not, you may need to use a mod like master controller to check the sim's relationships and make sure there isn't already a steady one in there.