"You need to opt in to be able to preview content. Press the button below to proceed."

Here's the message itself:

This message is not the actual problem: if pressing "Send to Messenger" button won't let you proceed (or you're seeing the box, but not the actual button), then you'll have to fix this real quick.

Thankfully, the solution here is pretty simple!

Chrome: go to Settings => Advanced Settings => click "Content Settings" in Privacy and make sure you're NOT blocking third-party cookies and site data. 

If you still wish to block them: click "Manage Exceptions" and type [*.] in a Hostname Pattern, then choose "Allow" in Behavior.

This should let you finish your opt-in process and use ManyChat to the fullest.

If that won't help, try to start a conversation with your Bot in Messenger and get a response from it. Doing something like triggering the "Default Reply" or any of the Keywords should work just fine.

Hope this helps!