"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, 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.

Hope this helps!