+r on some networks means you need to be registered to join, for example on Undernet. Other networks it is indeed +R, such as DALnet (and on DALnet +r means registered, but for a nickname).
Modes do not have globally known case sensitivity. +R does not HAVE to be +R around all networks, and it isn't. Judging from the message this particular user is getting, I would say they need to register with whatever username service the network has before they can join.
Seek help in the #help channel (or equivalent) of the network you use, it has nothing to do with mIRC.