Unfortunately, staff cannot unevolve pokemon so any evolution is final, nor can staff lower a pokemons level, so if it needs to level up to evolve and reaches level 100 there is nothing that can be done and it won't be able to. You will need to make sure every requirement for evolution is met before reaching level 100. (not including evolutions that need trade of evolutionary stones as these can be done at any level) It's recommended to evolve pokemon around level 97-98 to avoid any unexpected issues