Fixing Paypal error - Payment Processor Error message 10002: Security error Security header is not valid

Question

I found the solution to my problem before coming here, but wanted to share it so I'm posting my question first and then going to post the answer I discovered.

I setup Paypal pro for payments using my Paypal sandbox account, but was getting the following error:

Payment Processor Error message 10002: Security error Security header is not valid

See my answer for solution

Answer 1

What I discovered with the help of a friend who has lots of experience in CIviCRM is that I had set up the payment processor with the default URL for both LIve and TEst but used the SAndbox credentials only. By putting in the sandbox urls as the live urls. it fixed the problem.