PayPal Unauthorized API messages
Recent changes to operating environments have meant that PayPal Checkout on older osCommerce sites have stopped working and/or worked intermittently.
There are a number of steps to take to try and fix the issue:
Make sure you are using an update to date certificate file. Check /etc/modules/payment/paypal/paypal.com.crt
Make sure you are using the lastest version of cURL. Generally you need to be running PHP 7.3. We can patch your site to work with PHP 7.3. There is no need to upgrade to later version of osCommerce for highly custom sites.
Upgrading to the latest version of the PayPal App is also an option. We can integrate the current App into any version of osCommerce from MS2.2, Rc2A, MS 2.3. If you use Phoenix then we recommend you check the cURL issue.