Thanks for reaching out to the PayPal Community about this issue.
I know that would be frustrating. There are multiple reasons why this promotion may not have been provided to you or has not yet been issued to you. I would recommend reviewing the terms and conditions of the promotion for additional information. Please note that no one in the Community forum tell you specifically why this credit has not been applied. Check your cashback amount on http://www.paypal.com/myaccount/wallet. In case you have a business account with PayPal, you'll need to download the PayPal app, login and check 'Offers' section in the bottom part of the app homepage to find your cashback balance.