Log in PayPal developer account at https://developer.paypal.com using your usual PayPal login credentials.
Go to 'My Apps and Credentials' and select 'Live' (live keys).

'Add App name (it could be anything: e.g. your site name or business name) and click 'Create App:
Please remember that PayPal has its own restrictions for accounts from different countries. Sometimes you need to have business account (e.g. Russia), sometimes personal is enough (e.g. USA).
Once you've created an app, you can either enable testing mode or live mode.
Copy your 'Live' API credentials and paste them in your WP admin area.
Go to AliDropship -> Settings => Payments to enable PayPal as your payment gateway.
1. Disable Demo mode.
2. Change Currency to US Dollar ($). If you are from the UK, please set as British Pound Sterling (£).
3. Change integration method to Rest API
4. Enter your Client ID and Secret
5. Click 'Save changes'.

If you have any questions regarding PayPal sandbox settings, your account restrictions or settings please contact PayPal support.