Should this line:
var/mob/P=input(usr,"Select a Pokemon to Send Out","Select") in usr.pokemonlist+"Nevermind"
Be
var/mob/P=switch(input(src,"Select a Pokemon to Send Out","Select") in list(usr.pokemonlist, "Nevermind"))
All I did was add the switch statement and put the pokemonlist and "nevermind" in a list (which your original code still did but I like to see it that way for some reason)
Also I changed the usr to src, because you shouldn't call usr there. In fact, in all of that code change usr to src.
the verb does nothing :/. and thank you again for taking your time for helping me.