At a minimum you
- need to look up the error message in the online error messages manual, prior to posting
- you need to post your platform and your 4 digit database version
In a RAC instance each instance needs to have dedicated online redolog groups, called a thread.
You need to make sure (from v$log) your instances have separate redologs.
Secondly (the error message is already pointing to that), both instances need to have a thread enabled
--> alter database enable public thread <n>; -- usually 2