Actually there is no need for further details. The email notification and description was lacking relevant details about Kamailio version because of wrongly formatted markdown when the issues was created. I edited it to format it and it revealed that the version is 5.1.6, which is a no longer maintained version. The new rpc command was added in the supported versions, 5.3 and 5.4 at that time, being also in 5.5. You will have to upgrade.