Thanks for taking the time to open a bug report!

Describe the bug
Pretty simply; putting a player in when doing /itemjoin get doesn't work and gives an error in console.

To Reproduce
Steps to reproduce the behavior:

1. If a plugin/console runs it, returns an error
2. If OPed player does it, just gives the item to the player

Expected behavior
I expect not an error and to have the item given to the player

Screenshots
If applicable, add screenshots to help explain your problem.

Pastebin
https://pastebin.com/Ae0ykeG9

Please complete the following information:

• Resource Version and Build Number: v5.1.0-RELEASE-b545
• Server Version: Spigot 1.16.4

Additional context
Would appreciate if this was fixed soon, as it's important for me to give players certain items

Sounds like a duplicate ticket, pretty sure this is already fixed in the latest snapshot.
Link; https://ci.craftationgaming.com/job/ItemJoin/591/
Remember to remove your old ItemJoin.jar file

This bug has been officially fixed in the release of ItemJoin v5.2.0.
You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=383801

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