Why can’t I add more troops to my garrison?

I'm not sure if i have understood your question exactly but the Garrison option in the Garrison Building is there to let you specify which of your APC's (barracks formations) you want to participate in defending your city if it is attacked and in which order they will fight. You can change the order or select the small box to the right to add or remove them from defending.

You cannot manage the troops here though. To edit the troops that are selected for each APC open the APC menu by tapping the brown Truck icon to the upper left of the main screen. For each APC listed that you have unlocked you can tap the small cog icon to the right to then edit the troops and heros in that APC.