I don’t really get what you mean by that.
Do you have the check command definition?
For linux memory there already is a pre-defined check command mem inside the ITL. This works only locally, on the machine the plugin is executed.
There also is a pre-defined check command for snmp-mem, which needs the check_snmp_mem.pl added to the /usr/lib/nagios/plugins directory.
icinga2 daemon -C might also give you some hints/error messages.