Drupal 7 - Error on Checkout, e-Commerce
If you're new to Drupal commerce, try installing Commerce Kickstart.  There are detailed installation instructions available.

Drupal Commerce is fairly complex and there are lots of different types of errors that can result.  Commerce Kickstart is a package (known in Drupal as an installation profile) that sets up most of the commerce stuff for you.  If you're not experienced in working with Drupal, this is an easy way to get started without having to troubleshoot error messages.

If you do try Commerce Kickstart, do so from a new site.  If you try to install over Drupal Commerce it could get messy.