Could not add User to the Whitelist

The [Server] Server thread/INFO Could not add [user]to the whitelist error is most common with Minecraft versions that use the UUID system. If you encounter this error when trying to add someone to the whitelist, it could either mean the whitelist.json file on your server is corrupt or that the server doesn’t know that user’s UUID.

• To fix the corrupt whitelist file, simply delete the whitelist.json file from your server and reboot it, then try to add that user again.
• If the file isn’t corrupt, then it may just be a case of the server requiring that user to join the server before enabling the whitelist so that it “knows” them. Once they have joined your server once, try to add them again to your whitelist using the whitelist add user command.

In some cases, a similar problem may occur with opping a user as well. In that case, you would want to try deleting the ops.json file instead if it is corrupt