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 [*.]facebook.com 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!