GetAmped 2 will only accept .skin2 files; standard .skin files from the original game are not compatible.
If you are looking for a version of the game still actively supported, the Taiwan server is a common choice for international players. First skin I made after 8 years (need help) : r/GetAmped2 getamped skin free install
How to Get and Install GetAmped Skins for Free: A Complete Guide GetAmped 2 will only accept