Why isn’t this unit showing up on the preps screen?
There are a few reasons this could be happening. The most obvious is that the unit could be dead or removed from the party, but it’s pretty rare for that to happen unintentionally unless you accidentally erased them from the party by loading them as a red or green unit.
Less obvious, but more likely to be the cause: make sure they do not have the “Supply” checkbox set in the Character Editor or Class Editor. This checkbox does not grant supply access — that’s done from the patches list, or with a skill in SkillSys. It’s a leftover from FE7 Merlinus used to make him not appear in the preps screen. A character with this checkbox set will not appear in the preps screen.