If you've been having conversations with people from your FB page Messenger for a while, you may sign up for ManyChat hoping you can pick up the conversation where you left off, and immediately have all previous conversations loaded into ManyChat automagically. Unfortunately, that's not how it works.
We work closely with Facebook's terms of service, and one of the stipulations is that you can only message NEW subscribers through ManyChat (i.e. people who start conversations with your page AFTER you install ManyChat).
That being said, if you are clever, you can find ways to encourage people to re-open old conversations with you.
Sometimes it's as simple as going back into the FB page manager Messenger and following up manually on a conversation.
Hope that helps explain how this works!