Hyde here from Shopify.
While it's very difficult to diagnose the issue without having a look at your store, you're given a PayPal Express Checkout account with the email you used to sign up for your Shopify store as soon as you open a store.
So before you can collect payments for orders made with PayPal, you need to make sure you have the right email address connected and that you have a Business Paypal account.
I'd recommend having a read through our help document on it here, but if you have any further questions I'm happy to answer them.
All the best, Hyde.