Have this same problem. The sim in question has completed the serial romantic aspiration but during the wedding event ”sims’s fiancé isn’t around” even thought they are clearly there. Event venue is a beach.
I had this happen repeatedly while I was playing the black widow challenge.
What worked for me: if the fiancé is in your household, have them click the arch.
If the fiancé isn’t in your household, call them over.
As near as I can tell, the distance between the people to get married needs to be really small. I don’t know if that changed recently, but… it feels like it did.
