Why can't I add Pokemon into a Gym?

When adding a Pokemon into a gym, it says 'Error' and kicks me out. My Pokemon are at full health, I've tried with different Pokemon, but still nothing. Can anyone help?

Update: It's working now, however it's not the same as some other questions because I'm still able to use Pokestops and capture Pokemon. Just not gyms.

Solution 1:

I've had this happen to me several times. It is possibly due to someone else having placed a Pokemon in the gym before you while you were trying to place one.

This causes an error because the gym is now of a different colour or all it's spots are full.

Solution 2:

Restart the app. Chances are you'll find you can't place a pokemon in there anymore either due to the gym changing colors or another trainer on your team placed their pokemon in the open slot.