go to that skin location and make sure that skin is not inside other folders
To summarize this up,
Instead of have this below v
..\osu!\skins\(skin name)\(skin name)
have this one v
Instead of have this below v
..\osu!\skins\(skin name)
Instead of have this below v
..\osu!\skins\(skin name)\(skin name)
have this one v
Instead of have this below v
..\osu!\skins\(skin name)