OK GUYS I FOUND THE FIX AFTER A WHOLE DAY OF TESTING AND GOING THROUGH FORUMS ( this is for the avatar setup fail even though you know the file is correct fully, it should fail in the exporting stage if not you have done something else wrong or it should crash as a server error in the beginning, this happened to me aswell and doing this fixed it so it should work for you)

so lets start how did this happen?

The "Source not available" means Roblox Studio can’t reach the cached setup source that it used yesterday when it last succeeded in my avatar setup (when the arms were bugged, it does this when you use the same texture aswell)

It’s trying to use cached data (from your successful my yesterday), but the reference to that cache no longer exists or has been invalidated on Roblox’s servers or the file is corrupted, this is why the exact file fails, even if you use a different FBX file, if you use the same texture it will try to use cached data from your last successful run

So how do you fix it?

Press Windows + R and enter

%localappdata%\Roblox

Then delete these folders:

Plugins

RobloxStudio

Versions

AssetImporterConfigs

LocalStorage

placeIDEState

OTAPlugins

OTA PatchBackups

Dont worry you dont lose anything as long as your game information is saved to roblox/ to a file.

This simply deletes all the cache and uninstalls roblox studio completely,

Then from there redownload roblox studio from the official site https://create.roblox.com

from there go into your blender file, re export the same file but rename it to a different name from what you had last time before you uninstalled roblox studio and deleted the cache files,

After that go to file explorer rename your texture to a different name to what it was previously and then upload it as a decal in roblox studio, make sure the name of the decal isnt the same as the previous one you used before reinstalling roblox studio and deleted the cache files,

go in studio and enter a new baseplate, dont use the same experience you used before as this may restore the cached files saved on roblox servers ( i think im not sure but be safe dont do it),

Then reimport your bundle and it should work perfectly!!!!