Package Must Be Selected
After having created a listing, to be sent to PayPal, a package must first be chosen. I would recommend going to Theme Options > Add Listing Page > Listing Upload Options and where it says "Choose Package Required", setting this to Yes.
This means that anyone trying to access your "Add Listing" page that has not selected a package first, will be redirected to your packages page to select one before proceeding to create their listing.
PayPal API Keys
Before you can start processing payments you will need to get a PayPal Client ID and PayPal Client Secret. To get your API keys go to https://developer.paypal.com/docs/api/ and up the top right click "Log into Dashboard".
Once logged in you should be on the "My Apps & Credentials" page, scroll down to "REST API Apps" and click on the Button where it says "Create App"
Then create a name for your App and click "Create App". Then you will get your Client ID & Secret which you can paste into Theme Options > PayPal/Stripe > PayPal where it says "PayPal Client ID" and "PayPal Client Secret".
To test that everything is working, make sure that you are using the PayPal Sandbox. When you are ready to go live you will need to go to Theme Options > PayPal/Stripe > PayPal and select "Live" for the option where it says "Sandbox or Live".
Once having done this you will also need to go into your PayPal App and select "Live" instead of "Sandbox" through your PayPal Business Account. Now you will have new PayPal Client ID and PayPal Client Secret Keys which you need to use instead of the "Sanbox API Keys" you were using inside Theme Options > PayPal/Stripe > PayPal.