Run the following command to find if RMAN sessions exist in the database instance (format the output for readability):
s.SECONDS_IN_WAIT AS SEC_WAIT,
FROM V$SESSION_WAIT sw,
WHERE sw.EVENT LIKE '?ckup%'
If the sessions are "WAITING" for resources, then kill the sessions using OS command "kill -9" at OS level (prompt).
Some platforms include a command-line utility called orakill that enables you to terminate a specific thread. From a command prompt, run the following command, where sid identifies the database instance to target, and the thread_id is the SPID value from the query.
orakill sid thread_id
And restart the backup process.