When i try to backup of tablespace then this error occur.
"Ora-01123 cannot start online backup; media recovery not enabled"
how can i resolve this error

follow the below mentioned steps:-

alter system set log_archive_start=true scope=spfile (To enable automatic archiving)
shutdown immediate
startup mount
alter database archivelog
alter database open

Archive log list (you can check through this view if it is enabled or not)