TIP: If you are receiving a "User Authentication Error" (Result Code 1 or 26) when attempting Payflow Pro transactions, the first step is to make sure you have entered the correct vendor name and password in the geoAdmin. If you're sure of that, but still receiving authentication errors, it may be the result of IP address restrictions in your Payflow Pro control panel. To resolve this issue, try following these instructions:

Removing the allowed IP address within the PayPal Merchant account Manager. You can locate the allowed IP address list by going to account administration and clicking on Service Settings. Under the Payflow Pro heading, there is a link for allowed IP addresses. Remove the IP address (or adjust them to correctly reference your server) and attempt the transaction again.