Just to be 100% clear, I assume you are using the link from within that document, and I assume that if you hit "Cancel" you still cannot hit the "Get" button to download the El Capitan installer:
https://itunes.apple.com/app/os-x-el-capitan/id1147835434?ls=1&mt=12
If not, please try that. But assuming you will (or already have)...
I am suspecting the issue is that there's a problem with your iTunes account and poor old ancient Snow Leopard iTunes doesn't have the proper ability to show you a fully descriptive and useful/accurate error message in that regard.
So you need to make sure there is no issue with your iTunes (Apple ID) account in terms of billing as the message sort of (awkwardly) implies. Do you have an iPhone or another Apple device? If so, using the same Apple ID can you "purchase" any free apps from the App Store or iTunes on those other devices? This is the ultimate test to determine if there's any problem with your account.
For example, if there's any problem with your account then on any device you will not be able to purchase new paid apps, "purchase" new free apps, or update any current apps. Here, with iTunes in El Capitan you are trying to purchase the free El Capitan upgrade to get the download. This is why I suspect an account issue.
If you have an iPhone I would change or update your payment information and make sure you don't have any outstanding balance (https://support.apple.com/en-us/HT204088) and try to download a free app.
If all that checks out, make a new Admin account on your Snow Leopard, sign into iTunes from there, and try again to upgrade to El Capitan.