Hello,
I understand you are trying to create failover replica of a second generation SQL instance through cloud console. I tried to reproduce your issue but was not able to as I successfully created failover replica of SQL DB through cloud console. Can you please share the screenshot of error message?
Alternatively, You can create failover SQL instance through Gcloud command. Please follow step given here.
If you still get some error message, please sent me screen-shots of the error message.
When i tried to create a failover instance for my master db, it keep showing "Could not complete the operation".I have tried with many difference instance name but still no luck.I can create read-replica with no issue, any idea how i can debug this?