This article covers methods to resolve 'NRPE Command Plugin Not Defined' for our customers.
This error is very straight forward. Usually this is caused by a mismatch between the command name declared in Nagios XI to be check through NRPE and the actual command name of the command directive in the remote host's nrpe.cfg file.
This problem will occur in versions of check_nrpe before v3.
What is happening here is that the initial -c check_users is being overwritten by the -a -w 5 -c 10, as check_nrpe thinks the -c 10 argument is the command argument, not one of the -a arguments.