How to restore 11gR2 fullbackup to 12c database in rman
This is not going to work. Oracle binaries of a certain version correlate to the Oracle Database dictionary of the same version.

RMAN is a backup and recovery tool. It is NOT an upgrade tool. 

You need to restore and recover you database on 11g binaries. From there you can upgrade your database to 12c. 

You can have many versions of Oracle Database installed on the same host. The different binaries go into different ORACLE_HOME.