The code is incorrect or has already been used
The 6-digit two-factor code wasn’t accepted. Wait for the next code in your authenticator app and enter it. A lost authenticator can be recovered on the server.
What it means
The 6-digit code you entered when signing in or turning on two-factor authentication wasn’t accepted.
Why it happens
The code had already expired when you entered it, and the app had moved to the next one.
The same code was used a second time.
Your authenticator app has several entries and you read a different one. When turning on two-factor authentication, read the entry you added a moment ago.
What to do
Wait for the app to show the next code, then enter the 6 digits it shows now.
If you lost the authenticator and a browser is still signed in, replace or turn off two-factor authentication under Sign-in & security in Settings. Otherwise reset the password on the server, which also turns off two-factor authentication. Combos, exchange keys and history are kept. See Forgot your password or lost your authenticator.