Hello there, not sure exactly what you're trying to do, but if you want to use 2CheckOut to accept payments (including PayPal) on your Prestashop 1.6 site, you'll have to upload the 'twocheckoutpp' folder to your modules directory and activate the module. With respect to the 'fix', I didn't come across a patch or anything, just the 'twocheckoutpp' module which is probably an upgrade over the previous module (that was designed for 1.4 but also worked on 1.5).
Getting an 'unauthorized' first cc and going to an authentication page... my only wonder is whether you have successfully signed up for 2CheckOut and had your account approved for accepting sales. If you do have an approved account, then my second thought would be whether you've correctly configured the module's settings in your backoffice, (secret word, etc)
Hope that helps. (But FYI, the original reason for this topic was to find out how to get the 2CheckOut module to land on the Order Confirmation page but that has not been solved. It seems the module was not originally designed to land on the Order Confirmation page and I haven't yet found a modification.)