I am having this issue as well but it seems to be random. For one of my sims she could plan a stayover with her boyfriend from the relationships tab sometimes but not the phone. With another sim he hasn't been able to plan a stayover with his girlfriend at all. I end up getting another family member who's not in a romantic relationship with the sim to plan the stayover. I do have mods installed.
This is happening for me as well now, I can’t invite my sims parents or one sibling, nephews are fine but her siblings and parents are unable to be invited for a stay over
