Here are some common reasons why you may have trouble connecting:
Logged into multiple Google accounts
If you’re logged into multiple Google accounts at once Google may be fumbling when trying to authenticate you. Try either logging out of all your Google accounts or try opening an Incognito Window in the Chrome browser which will give you a window where you’re not logged in to any accounts. Then try connecting with Replybutton again.
Gmail not fully set up
It’s possible to have a Google account, but not have Gmail fully set up. Try visiting gmail.com while logged into your Google account. If you’re using G Suite, your IT admin might need to enable the Gmail app for you.