Hey there, TMB
Sometimes when the Microsoft Authenticator has already received a request to verify your login with a code, if you were to generate a new request while the request is active, this error will show.
So, for example, if OneDrive on my computer is displaying a code for me to verify in the Authenticator app and I decide to try signing into Outlook while OneDrive is waiting for me to verify my login using my Authenticator app. When Outlook tries to generate a code for me to verify in Authenticator, it will generate me an error message just like yours above.
This is because the Authenticator app can only process one request at a time. Please can you make sure that no other app is wanting you to verify a code using the Authenticator app. Either on your current device or another device you may be using.
The Authenticator app generally times out after about a minute. I often run into this problem at work when Teams and OneDrive both start at similar times and starts trying to authenticate me.
Hope this helps,
Nathan