Well thats strange, ik the video is a bit old but i tried looking it up and it seems to be the same process. Ensure the pokemon knows hidden power and you select use on the ×20 gems. If it still doesnt work idk why, will look into it
Ok I think I found the problem, I was going through the update notes and came across a part where mysterious gems produce double the amount of type gems now but Hidden Power changer cost has been doubled meaning it would require 40 gems now, not 20. Hope this helps