By default the plugin should create some pages, one of which is the Shopping cart page. This should be done automatically, but to check follow these instructions:
Check your pages for the existence of a page entitled 'Shopping cart' and note down the id number if there is one. Then go to the eShop settings pages, specifically:
and check that the Cart id is the same. If it isn't then add it.
If the page doesn't exist you'll need to create it, along with some others. Page title can be anything, but these are the defaults, followed by a shortcode that needs to be in the content:
Thank you for your order:
Alternatively, a deactivate/activate should fix the problem.