Not sure if it has been mentioned, but the plugin Email subscribers and Notification that you have as working in V1, dosn’t look like they are going to support classicpress.
I messenged them on facebook as I was interested in their starter plan at a special low price, as I currently looking for a plugin like this either free or a small nominal price, and the response was this :
*"Malay Ladu At this moment, there is no plan to support Classicpress. *
Did you check “Classic Editor” WordPress plugin which remove block editor and add the classic editor with the latest version of WordPress?
So the search goes on, and told them this as well that i wont be purchasing from them.