5. In the Payment Gateways screen choose PayPal Checkout payment processor and add your Paypal client id.
Note: If you are using sandbox credentials please enable Use Sandbox option.
Extra: You can enable the pay now checkout flow for using Pay Now Checkout Flow option.
Where to find your PayPal Checkout credentials?
In your PayPal account, go to Dashboard -> My apps and credentials and copy your client id from your PayPal app.