Backup file manipulation operations must be serialized

This error comes up when trying to perform and operation (such as a backup or database shrink), while another similar operation is already running.

Backup and file manipulation operations (such as ALTER DATABASE ADD FILE) on a database must be serialized. Reissue the statement after the current backup or file manipulation operation is completed

If you cannot wait for the job to finish or it is running in error, you can execute this command to find what is actually running:

SELECT session_id as SPID, command, a.text AS Query, start_time, percent_complete, dateadd(second,estimated_completion_time/1000, getdate()) as estimated_completion_time
FROM sys.dm_exec_requests r CROSS APPLY sys.dm_exec_sql_text(r.sql_handle) a
WHERE r.command in ('BACKUP DATABASE','RESTORE DATABASE')

This will produce the list of jobs:

Locate the job in question and kill it:

KILL 58; 
GO

The process is not clean, so use it only if you definitely want to kill the backup job.