If under Settings -> Notifications you see this note (see screenshot below) instead of the 'Subscribe' button, to troubleshoot it you have to follow the instruction from the Note. Head over to your Facebook page connected to that ManyChat account, view it as a visitor, and message it. After this, you will be able to subscribe to Messenger notifications.
In case you don't see the note at all, but instead see this (screenshot below), this means that the button can't be displayed due to some browser issue. Try disabling all ad-blocking plugins you have in your browser. If this doesn't help, try clearing the browser cache or opening the page in another browser.
If the button still doesn't show up, submit a support ticket mentioning what troubleshooting you tried, and we'll take it from there!