When Issuing an update statement I am getting the following error

ERROR at line 1:
ORA-01562: failed to extend rollback segment number 5
ORA-01650: unable to extend rollback segment RB2 by 32 in tablespace RBS

The Rollback segment is as follows

RBS - TableSpace - Initial =1M, Next = 1M Maximum value = unliminted

RBS Datafile = Size = 500MB (Used = 4MB)

Rollback segments = RB0 , RB01,RB02,RB03,RB04,RB05,RB06,RB07
For all of them
(Extents )Initial = 256K,Next = 256K, Optimal = 512K

Minimum Number = 2, Maximum Number = 32765

As I see there is a lot of space to extend then why is it giving the error