If you disable this setting then customers won’t see Paypal choice when booking.
Otherwise, if you activate this choice you must create a new App on your Paypal account. You can create a new Paypal app by visiting PayPal application page
Enter client ID of your creating App and the client’s secret to appropriate areas in these settings.
Try and save Sandbox from Mode if you want to test the Paypal processing engine. After the test, you can change the Sandbox mode to Live mode.
The currency you choose in Settings> Payments must be supported by Paypal!