why can't i add pokemon to a gym

Maybe you have one in already?

Also a known bug...Niantic on it.

I have that issue sometimes. You can try to get out and then re-enter the gym and put your PKM there. Or maybe you have to wait for 10 minutes after someone else just put in their PKM :(

Was the gym being attacked?

Yesterday I decided to fight down a fully charged gym in the middle of the night. When I was setting my lineup for round two I saw a other guy across the street with a power-bank wired to his phone. I knew he was a trainer and was hoping that the guy would join in and help out or do something, but he just stood there across the street with his phone glued to his face and did nothing. I continued the fight with my stranger near by in the middle of no where and took the sucker down by myself. As expected, got sniped as soon the gym became neutral. My asshole wierdo of a team-mate added in his top tier Blissey and took off right after.. I mean wtf. Got so salty when i found out I had to wait like 10 min before I could add in my Pokemon...

When a gym switches teams/colours you have to wait 7-8 minutes before you can add your mon to it.