"20&22" is not a number. "20" is a number, "22" is a number, but "&" isn't. That's your problem.
An evolution method can only have one custom value. In this case, either level or map number. You can't write both into pokemon.txt.
The solution is to copy the "Level" method, and in the code stick the phrase: && [22,23,24,25].include?($game_map.map_id). You define the level in pokemon.txt, and the map numbers in the array (square brackets) in that red bit of code. The slight drawback is that you can't use the same evolution method for evolving in different places (e.g. Ice Rock and Mossy Rock) - you'll need one method for Ice Rock evolutions and one for Mossy Rock evolutions.