Can't put pokemon into gym, need help

(upload://uTqVOC0stJIVmtC5AtiMbzW6ZKB.jpeg)

As picture shown, everything else is fine, i can do poke stop, catch pokemon as normal, but when i approach a gym, when i click Abd show no pokemon to put in. If i need to battle a gym, there is no pokemon to battle as well. Anyone know if this is a bug or something. Thank you

Likely a bug. Also wrong section pls move