As of August 15th, 2019 - Facebook will be discontinuing usage of the Buy button on the Messenger platform. This feature can only be used until that date.
It's currently possible to receive in-bot payments using Stripe or Native Messenger Payments.
You'll be able to access the integrations from the Configure tab of your bot.
Once you click the "Run Wizard" button, you'll see the options to set up one of these two services
*Please note that, if neither of these two payment services is available in your country, you would need to set up a payment system of your own (with a payment processor available in your country). These transactions would need to happen on your website, landing page, or a similar platform.
Once you've followed the setup wizard and you have your integration of choice arranged, you can go into your Automate tab, add a Gallery card, and then pick the "Buy" button
Once you've set this up, your Buy button also has some Advanced options that you can add - like sending specific blocks after a person has completed the purchase, the data you want to collect, attributes you want to assign.
If you set everything up, and it doesn't seem to be working - the first thing you should try is remove the integration entirely and then start over.
If that doesn't help, you can report this to the support team - firstname.lastname@example.org