The game won't let my sims marry?

Hello! I am doing the not so berry challenge and I am trying to finish the "leave someone at the alter" task. However for somereason I can't start the wedding? When I click on the arch both the "plan wedding event" and "get married to" options are grayed out and the reason is that my engaged partner isn't on lot. apparently this is a common problem  as I've seen other people say the same thing. I've tried the fixes that people recomended but nothing is changing the options. Normally I would just bypass this and have them elope but seeing as I need to leave someone at the alter for the challenge I can't really skip over this one. 

Any advice?