I'm using ItemJoin v5.0.5
Using paper 1.12.2
So whenever I try to run the plugin, the console will return with having trouble enabling the file or something.

items.yml: https://pastebin.com/euWwmeBN
config.yml: https://pastebin.com/2SYyDUMN
en-lang.yml: https://pastebin.com/CYN1hZdL

console message: https://pastebin.com/DsRkcgQ5

Interesting.. everything looks to be proper.

Can you try the latest snapshot and let me know if this resolves your issue, please?
Link; http://ci.craftationgaming.com/job/ItemJoin/361/

Used the latest snapshot, still doesnt work :/ Console returns with the same error message.

Using the exact same version as you;
This server is running Paper version git-Paper-1617 (MC: 1.12.2) (Implementing API version 1.12.2-R0.1-SNAPSHOT)

And I cannot seem to replicate this error. Either it is something specific with your server configuration or something is corrupt with your server jar file.

You don't have multiple ItemJoin.jar files in your plugins folder correct?

correct

Okay, so I think I am starting to narrow things down. Does this error message occur on START or are you executing an ItemJoin command from the console window that causes this error to occur? If you are trying to execute a command, can you list it, please.

Occurs on start.

Okay, so a couple of things.

1.) Try the latest snapshot; http://ci.craftationgaming.com/job/ItemJoin/385/
2.) If the error still continues please send me a startup log file so I can see the process in which the error occurs.

3.) (If the error is still occurring); I did some talking with other developers and this error does not seem normal. There is definitely something missing and it looks to be the bukkit library that PaperSpigot imports. It is highly likely that your downloaded jar file of paperspigot is corrupt (It's hard to believe but a simple flipped bit on a jar file can cause only minor errors to appear that we are unable to control or fix ourselves). Just double-checking with a fresh jar download wouldn't hurt; https://papermc.io/ci/job/Paper/1618/

Let me know!

Hey I haven't heard from you in a while, was this issue resolved?

Marking this as inactive, please reopen this if the issue persists.

ItemJoin v5.0.6 has been officially released.
You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=320427

Please read the changelog as the documentation has not been updated yet;
https://github.com/RockinChaos/ItemJoin/wiki/Recent-Changes