in order to accept payments through PayPal, you must create and enable a new REST API app, as explained here
Once you have created your app and tested the integration, you can enable live payments by inserting the Live Client ID and Secret, and enabling the "live mode" below.
IMPORTANT: if you are going to accept payments in a currency you don't hold, please make sure to enable automatic currency conversion.
To do that, go to PayPal dashboard and select Profile
-> Profile and Settings
-> My Selling Tools
-> Block Payments