To configure Braintree:
- Install, Activate, & Enable Braintree Extension
- From Manage, go to Store > Extensions
- Select Payments from Category in Store Extensions section.
- Find Braintree and Click Settings Cog Icon
- Click Test if you're working in a sandbox mode; otherwise, Click Live if you're ready to go live.
- Select Automatically Capture Payments to authorize and immediately capture payment for each processed transaction. Select Manually Capture Payments (Authorize Only) if you only want to authorize payment and would rather manually capture funds at a later time.
- Provide your test or live credentials: Merchant ID, Public Key and Private Key
- Click Update
How do I find my Merchant ID, Public Key and Private Key
See: https://developer.paypal.com/braintree/articles/control-panel/important-gateway-credentials