Hey again @MaxMunch
Once you're on the Payment Methods page as mentioned before, you need to click on the blue button " + Add Payment Method" then on the next screen, choose "Add Your PayPal Account" (1) and you should be presented with a PayPal Login popup window (2). After login into your PayPal account and allowing automatic payments for Sonic Academy, click on the green bar "Add Payment Method" (3)
You can also check your PayPal account to see if Sonic Academy is listed in your authorized automatic payments to check if the PayPal process went correctly through.
At checkout ( once you've got something in your cart ), make sure to select the correct method of payment before clicking on "Purchase", you should see your PayPal email. Alternatitively you can also use your PayPal Balance.
If you're still having issue, please reach customer services via mail : firstname.lastname@example.org