when I logged in I was able to use headbutt on a tree which I did and got a ekans as a battle encounter after that I went to heal and then proceeded to combat my friend in a PVP battle after losing I was sent to the pokemon center and now I can no longer headbutt trees even though my pokemon Which is a Pikachu knows headbutt I've tried multiple trees and still can't headbutt even tried Re-logging any luck

when I logged in I was able to use headbutt on a tree which I did and got a ekans as a battle encounter after that I went to heal and then proceeded to combat my friend in a PVP battle after losing I was sent to the pokemon center and now I can no longer headbutt trees even though my pokemon Which is a Pikachu knows headbutt I've tried multiple trees and still can't headbutt even tried Re-logging any luck

Losing battles reduces happiness. Although I'm not sure if this is the same for PvP battles. Nevertheless, check if your Pikachu has 150+ happiness by typing "/happy 1-6" in the chat box where 1-6 is the position of your Pokémon in the party.