Error message saying to include my merchant ID on the settings page

When you make a button, there is a place for you to enter your Merchant ID. This is a feature typically used if you want to send payments into another account. This field is optional.

On the plugins settings page, is the main location where you want to enter your Merchant ID. This field is required.

If you are seeing this message, then that means that you only entered your Merchant ID in the button but not on the settings page.