PROBLEM:

While dropping a redolog group , got below error.

SOLUTION:

First get the status of redolog group:

We can see the though the redolog group 1 has been archived, its status is ACTIVE. I.e this redolog is required for crash recovery or block recovery.

So to change the status from ACTIVE to INACTIVE, switch logfile multiple times, till the status is INACTIVE.

Once the redo log group status is INACTIVE, drop it.