Sim can’t marry

Problem

My Sim can’t marry although he’s already in a steady relationship and the relationship is all the way up.

Solution

If you already have 8 Sims on your lot (or a pregnant Sim that could fill it to 8) and your fiancĂ© is not living with you yet, then you can’t get married. The game assumes that the new Sim is moving in with you and not that you move in with the other Sim.

Further Reading