Can't play as one of my household sims

imlividd​  I'm glad you fixed it, and an MC reset is always an early troubleshooting step for me.  For future reference though, if you moved the sim in by inviting them to visit and asking them to move in (or using the Add Sim cheat), that could be the cause of the problem, and it's best to use a different approach.

The problem is that tourist sims visiting from other worlds are really mini-sims: they lack the full set of data that resident sims have and that the tourists have in their own homeworld.  So when you make one of these tourists a resident, the game has missing data to try to create and doesn't always do a good job of it.

After the fact, an MC reset with ErrorTrap in place is the best way to fix it.  To avoid it, either move the sim into your active household while you're playing within their homeworld, or when you're in your sim's homeworld, use MC's Add Sim command on an empty house or lot to add the foreign sim.  Either way transfers all the sim data properly.