With your recent trade with the Reds, you received 3 players and traded away 2 players. That trade, along with this move gives you 40 MLB players and 18 MiLB players.
In order to add Caleb Joseph, you need to make a roster move to open up a spot on your MLB roster. With that, you just won two more free agents, Sergio Romo and Andrew Bailey, which means you now need 3 MLB roster spots to add them to your roster.
You need to post the needed moves before any of these players can be added to your team roster.