To use Paypal for Elementor Pro Form, you need a PayPal account obviously to receive the payment. In order to connect it with your form, you'll need API access which you can specify in Dynamic Content > API > Paypal
To create a new API, you need to login into your Paypal developer dashboard and navigate to My Apps & Credentials.
In My Apps & Credentials, you can find your existing APIs and create new API by clicking Create App button.
Now in the next option, you can specify the App Name, App Type (whether Merchant or Platform), Select Sandbox Business Account and create the app. Once the App is created, you will find its API Credentials on the My Apps and Credentials page.
Now in your WordPress Dashboard, you will need to input these API credentials in Dynamic Content > APIs