Why can’t I see the opt-in box on my store?
A shop can only have one Messenger app enabled, otherwise they interfere with each other.
If you can’t see the opt-in box, you might have more than one Messenger app installed, or the Shopify Messenger channel is enabled.
1. Check for apps and delete any unused ones.
2. Check if the Shopify channel is enabled. If it is, turn it off.