[Trexmaster]

You can only hold up to 3 spirits, even if you go past that many ranks in Possession, and releasing spirits after hitting your cap will lock you out of interacting with spirits (your only option when interacting with them will be 'Nothing' rather than 'Release' and 'Possess' being available at all) until you use a Fruit of Talentlessness.

[Shujin]

huh this is strange, I had 5 spirits on mine character.

[Neus]

You might not have had enough mental stamina. Possession requires 10, release requires 5. If you don't have enough the option won't show up.

[Esther]

Perhaps showing the option would be better, and just having it return a message like "you're too mentally fatigued to do this right now."