No matter what type of url I use, like https://static.planetminecraft.com/files/resource_media/skin/skinseedskin-1646852037483-15482743.png or anything like that, it warns me saying "You have to provide URL, player name ...."

In server logs, I see:

[FabricTailor] An error occurred: https:/static.planetminecraft.com/files/resource_media/skin/skinseedskin-1646852037483-15482743.png

I see the extra forward slash being removed, no matter what I try. I think its a bug on your end, maybe like youre convert the string to a file url, which is known to remove double slashes according to UNIX file urls. Maybe have a priority based if else condition that specifically checks for "https://" startsWith case, and straight away download via that way.

Tested with command: /skin set URL classic https://static.planetminecraft.com/files/resource_media/skin/skinseedskin-1646852037483-15482743.png

Please check if you used the correct syntax:

• /skin set URL classic (url here) for full-width/steve skin
• /skin set URL slim (url here) for slim/alex skin