400 status code error
If you are got http response code 400 when accessing https //api.sandbox.paypal.com/v1/payments/payment
#1 Regular Price is required
Make sure that Listing Plans -> Your Plan -> The Regular Price is not empty
#2 If you are using the Recurring Payment Method, make sure that your Listing Plan (Listing Plans -> Your Plan) -> Period Day is not empty

403 status code
If you face the following issue: http response code 403 when accessing https://api.sandbox.paypal.com/v1/payments/billing-plans/.
Please follow these steps to resolve it:
- Go to https://developer.paypal.com/
- Log into your site account
- From the Dashboard sidebar, click on My Apps & Credentials
- Scroll down to REST API apps, Click on your App
- If you are using Live mode, please switch on Live area
- Scroll down to LIVE APP SETTINGS / SANDBOX APP SETTINGS
- Check on: Accept payments, Invoicing

