Sometimes when you try to transfer a Pokemon to The Professor in Pokemon Go, you are unable. If you are unable to transfer Pokemon to The Professor, you can fix this by closing the Pokemon Go app and reopening it.
Typically you are unable to transfer Pokemon because there’s an issue connecting to the Pokemon Go servers, restarting the app fixes this. If you don’t know how to restart an app on your phone (Google it or), just restart the phone.
If you are still unable to transfer the Pokemon, troubleshoot the issue as an internet connectivity problem (try things like turning off your WiFi or connecting to a different WiFi connection.