Disclaimer: not an expert at all
Have you configured emails for members yet? You need to configure two sets of email settings for Members before anything like that works. I discovered this the same way myself yesterday. If not this was a super helpful post from someone who knows what they’re talking about: Local Ghost 3.0 - subscribing does not work - #27 It’s an older version of ghost but the instructions still hold
If you have configured emails already I have no idea 
But if you read to the end of that post Hannah tells you how to write your help post (like the one I’m answering now) to be most answerable 