If your Commissary’s capacity for troops is full, you restricted from actions on increasing troop number.

Upgrading the Commissary allows you to hold more troops, and 

this in turn allows you to train more troops, heal troops, and convert prisoners.

Try after upgrading the Commissary!

■ How to Upgrade the Commissary

- [Main Screen] > [My Dominion] > [Commissary] > [Upgrade]