These errors usually mean that either you entered your transaction key incorrectly OR your account is set up incorrectly with PayPal. If you are sure you have the correct key and have tried WebFill a few times, please contact PayPal Merchant Support team at: 

https://www.paypal.com/contactus

Click Contact By Phone to speak with someone, or you can email them at vt@paypal.com. Tell them what error you are seeing when trying to process transactions through their Website Payments Pro API using Swipe. They usually just have to flip a switch on your account and you're good to go!

Note that from our experience, error 10501 always means that PayPal has not "activated" the "Website Payments Pro" portion of your account. So even if Virtual Terminal works, iSwipe will not work and report this error. Telling them you are getting this error when trying to process through WebSite Payments will allow them to fix it.

Quick tip: as a rule, the PayPal customer support minions won't know what "iSwipe" is, so it's often easier just to tell them you are setting up a "custom shopping cart" and getting those errors. Mentioning the word "iSwipe" will just confuse them and get them stuck in the "I'm sorry, I can't help you with that platform" loop.