If a player doesn't have command authority, first see which permission node the plugin is actually asking for. The visible command name and the permission node do not have to be the same.
Step by step
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Reproduce the issue with a non-OP test account. Check that the player is in the correct group and on the correct server.
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Start LuckPerms verbose recording. Run the command once and see which permission node is returned, in what context and with what value.
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Turn off verbose after recording is complete. If the node is wrong, go back to the target plugin's documentation, if the context is wrong, correct the group assignment accordingly.
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Test again with the same player and world. The
falsevalue from a group or narrower context may change the outcome.
Keep in mind
If the proxy and the backserver use separate LuckPerms instances, change where the permission is queried.