Could not parse server version string: 1.21.11 Unobfuscated

[03:20:39] [Server thread/ERROR]: [TuffX] Could not parse server version string: 1.21.11 Unobfuscated
[03:20:39] [Server thread/ERROR]: [TuffX] Failed to find mapping-1.21.11 Unobfuscated.json in plugin resources!

what does mapping refer to? sorry, haven't developed for mc before, only python webui's.