Some connection is using the database. This is nothing to do with cluster. Find out the connection and disconnect it. To restore a database on the existing database, it requires an exclusive lock. Since some users are using that database, SQL server fails to acquire an exclusive lock, which causes the error.
To find the session that is using the database:
select spid, status, loginame, db_name(dbid), cmd from master.sys.sysprocesses where db_name(dbid) = 'DB_NAME'
Find out the activity that is going on the database
DBCC INPUTBUFFER(<SPID NUMBER>
Disconnect the session:
KILL <SPID NUMBER>