Belicar/Nisa's romance is differently than the others. Their romance route is tracked through romance points since you can't lock in their romance yet. To be able to pick those choices, you need to at least have 3 romance points with them. It is done this way to separate those who want to romance Belicar/Nisa to those who just want to experience the main story without romance.
I'll revisit all romance choices with Belicar/Nisa and make them clearer so there's no confusion. Maybe by adding a heart emoji or something like that.
Thanks for letting me know so I can improve it! And don't worry, there will be more opportunities to increase Belicar/Nisa's romance score!