Enable external SMTP providers for G Suite
"You must send through example.com SMTP servers when you send as firstname.lastname@example.org. However, this functionality is not available for your account. Please contact your domain administrator for more information."
If your G Suite account has external SMTP providers disabled, you'll need to enable it before you can "send mail as" with Sidemail.
This link will only work if you're an admin on your G Suite account: https://admin.google.com/AdminHome#ServiceSettings/notab=1&service=email&subtab=filters
Once on the admin page, enable "Allow per-user outbound gateways". It usually takes 5-10 minutes for it to take effect for mail accounts.