to fix the error, edit the universal email server behavior, on the general tab, make sure a character set is selected.
To be able to create a download center, you will need to have your customers login using security assist before preceding through checkout.
The orders table needs to capture the logged in users ID number.
See the tutorials for "Applying Register/Login to your checkout", "Creating an Order History page" and "Creating a Digital Goods Download Center"
You need to progress through applying login and crating the order history before getting to the download center.