The most common cause is the time on the device being out of sync.
Check the phone settings and ensure it has time set accurate to the second or better yet that is synced with a time server. Follow these steps to re-sync:
Another common reason is clicking the wrong website, Google Authenticator lets you manage many separate 2FA tokens, please pay extra attention to make sure you are using the correct token.
The tokens expire quickly so also make sure you are entering the token quickly so that it doesn't expire.
If you need further assistance, please submit a request (top right) to contact support.