This is the message itself:

This message is not an actual issue: If clicking the “Send to Messenger” button does not allow you to continue (or you see a dialog box but not the actual button), this issue needs to be fixed quickly.
Fortunately, the solution here is very simple!
Chrome browser: Go to Settings => Advanced settings => Click “Content settings” under “Privacy” and make sure you are not blocking third-party cookies and site data.
If you still wish to block them, do the following: Click “Manage exceptions,” and then type [*.] facebook.com, and then select “Allow” under “Behavior.”
This should allow you to complete the selection process and make the most of ManyChat.
If this does not help, try conversing with the Bot in Messenger and getting a response from it. Triggering actions like “Default Reply” or any keywords should work normally.
Hope this helps!