When configuring MongoDB Ops Manager (MMS), MMS backup check the oplog of last 24 hours. If there is a oplog windows does not cover minimum 3 hours, Ops Manager will not allow you to continue the backup configuration:
Insufficient oplog size: The oplog window must be at least 3 hours over the last 24 hours for all members of replica set
Solution: If the current oplog size is indeed insufficient for 3 normal hours operations for your application, increase its size following this guide: http://docs.mongodb.org/v3.0/tutorial/change-oplog-size/
If it is the case that you just set up the DB and do an intensive insert, then you don’t need to increase the oplog. Wait for another 24 hours and try the backup configuration process again (https://jira.mongodb.org/browse/MMS-2374)