Recently, Google updated the permissions screen for connecting or reconnecting email accounts using the Gmail Single Click option in Reply.
Previously, sending permissions were selected by default. Now, you must manually check the box to grant sending permissions for a specific email account.
In order to reconnect your account with sending permissions please follow these steps:
Log in to the affected Gmail account, click on “Manage your Google Account,” navigate to the “Security” section, and select “Your connections to third-party apps & services.” or click here
Locate the Reply app in the list and click on its name.
3. Select the option to remove all connections for the Reply app.
4. Go back to the Reply web app, open the “Email Accounts” section, and click the “Reconnect” button next to the email account in question. Follow the prompts until you reach the permissions screen. Be sure to check the box next to “Read, compose, send, and permanently delete all your email from Gmail.” Then click “Continue.”
This will successfully reconnect your email account with the necessary permissions.