Why can't I send a follow-up with not replied condition?
You get three follow-up conditions to choose from when you send a follow-up to your recipients: Not opened, Not replied and Been Sent.
Not replied Condition is supported with Gmail, Gsuite, Office365, Outlook, Hotmail, Live and Exchange.
So if you have any Email Service Provider other than these connected using SMTP, you would be able to use only 2 follow-up conditions i.e. Not opened and Been Sent.