Passing email ID as username is valid for Sign in process when using Cognito for MFA authentication?

I am using AWS Cognito in my application. I am providing email ID as username. I am able to do registration process for my application. But, while signing through API getting following error: Previously, I was using username for registration & login. So, its working fine. Now, we have created different user pool in that added 2 client apps 1 which having secret access & another 1 which don't have secret access which we are using through front end for MFA after login process. So, by using this user pool I am getting this error. (Note : Using same Secret & access key for both. Because, both user pools are from same account.) So, Now, I need solution on this, is email ID not valid to set as username in registration process?
How can I use email as username?