When you go to checkout, the page goes to error. On reviewing the logs, you see the following recorded:
-----------------------------------------------------------------------------
>> [date] [time]
>> [client IP address]
>> DESCRIPTION:
System.Exception: Error loading payment gateway list
at _Checkout.Page_Load(Object sender, EventArgs e)
at System.EventHandler.Invoke(Object sender, EventArgs e)
at System.Web.UI.Control.OnLoad(EventArgs e)
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint,
Boolean includeStagesAfterAsyncPoint)
This happens because there are no payment gateways set to 'ON'. In the back end, go to 'Configuration > Payment and Shipping Gateways'. This will list the various plugins and their status. There are four possible settings.
ON - the gateway is turned on, and hence available to all users checking out
OFF - the gateway is off, and not available to any users checking out
TEST - the gateway is in test mode, and is only available at checkout if the user is also logged in as a site admin (in which case, the Kartris button will show in the top left corner of the screen)
FAKE - the gateway is in 'fake' mode, and is only available at checkout if the user is also logged in as a site admin (in which case, the Kartris button will show in the top left corner of the screen)
The last two are hidden from normal users but displayed to admins by design. This is so an admin can add and test a new payment system on a live site without having customers start using it before it is ready. The manual covers these options in more detail.
To clear the error above, you must have at least one payment system available for the user who is checking out. That means status set to 'ON' for a user who is not logged in as an admin, or set to 'FAKE' or 'TEST' if you are logged in as an admin.