I think we should print the reason of the failure before printing the usage

The default branch aborts on the <plugin-name>:<command> form, which is the third invocation advertised in the top-level help just above (and arguably the primary purpose of nextflow plugin).

For example nextflow plugin nf-hello:greet -help parses args = ['nf-hello:greet'], command.help is set, so Launcher.checkForHelp() calls usage() → usage(args). args[0] is neither install nor create, so this default throws Unknown plugin sub-command: nf-hello:greet — i.e. asking for help on a documented, valid invocation produces an error claiming it does not exist. (Execution in run() is fine — it handles the : form via args[0].contains(CMD_SEP); only the usage path aborts.)

Suggest falling back to general usage for the : form rather than aborting:

default:
    if( args[0].contains(CMD_SEP) ) {
        result << 'Execute a plugin-specific command'
        result << 'Usage: nextflow plugin <plugin-name>:<command> [args]'
        result << ''
        result << 'See the documentation of the individual plugin for its available commands.'
        result << ''
    }
    else {
        throw new AbortOperationException("Unknown plugin sub-command: ${args[0]}")
    }

This keeps the genuine typo error (e.g. nextflow plugin instal -help) while not blowing up on the legitimate plugin-command form. Worth a test pinning the behavior down.