If you don't have enough players on a MinL team before spring training, and you try to add players, you get an error message until you add the 20th player.
So you can't go from 12 to 13 or 14. You have to get over 20.
Workaround it to queue up a lot of pending transactions for several days until you get over 20. If you do that, it's hard to see who's on what's team, if you still need more SP, etc.
It's one of the first things new players will have to deal with. I don't see the sense in making something needlessly frustrating the 2nd day someone plays the game.