Yeah I have the same problem, no spell shows up when I spend a level on it.

Looks like a bunch of spells only appear when the colonist is drafted, including regrow limbs

i thought the same thing but then i realized it takes a while for the limbs to start growing back again, though i also discovered that limbs will not regrow if you have a prostetic, so if you ended installing a peg leg on one of your colonists you will need to remove it and then cast regrow limbs to have their foot restored