Hello, I wanted to join a mushroom battle, but when adding the pikmins the “go” button didn’t activate, it said that that mushroom needed 20 pikmins to play, but 
I can’t add 20, cause there’s a limit of 15 that I’m able to send. Does it have something to do with the level I’m at (I’m at 11) and I’m limited to how many pikmins I can send to mushroom battles? I don’t remember the specifications of my level 
About the flowers: after some time the flowers I’ve planted disappear? I’ve noticed my house doesn’t have some species I’ve planted
I don’t remember the requirements from 11 but perhaps you just don’t have enough of that specific type of pikmin? Battling more mushrooms than one requires more pikmins to go around.
Yes, flowers in the map disappear over time 
You’re right, it’s because of your level. You will be able to send 20 when you reach level 12 so you won’t run into this problem after that. The rain dot this is because if you don’t send enough Pikmin they won’t be able to carry back the rewards.
Please refer to the Mushroom issue of official help center.
Mushrooms — Pikmin Bloom Help Center
“You must also send at least 10 Pikmin to participate in the regular mushrooms and at least 15 Pikmin to participate in the large mushrooms.”