Some Trainers may experience a bug where they are unable to assign a Pokémon to a Gym shortly after the Gym was claimed for their team. Attempting to assign a Pokémon returns the error message: “The Gym is under attack! Try again later.”
One of the root causes of this issue has been resolved in Pokémon GO version 0.67.2 (1.37.2). We’re actively working on a fix to completely resolve this issue.
We believe this issue is fully resolved in Pokémon GO version 0.69.0.