A plugin was built for a newer server version
Applies to Paper, Vanilla.
What it looks like in your log
org.bukkit.plugin.InvalidPluginException: Unsupported API version 1.21
What causes it
A plugin declares an API version newer than this server supports.
How to fix it
Update the server to the version the plugin targets, or install the plugin build made for the version you run.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log