Here is one sample scenario for ORA-19811 error. I connected to target as / and tried to execute the change command. It is giving the ORA-19811 error. You should connect to recovery catalog database to execute the change command.
Also, you cannot use the KEEP for the backupset in Flash Back Recovery area (FRA). You need to copy the backupset outside of the FRA location and use the catalog backuppiece command whenever you wanted to restore it.