I am glad you are finding the article useful and thank you for your response. I will do my best to help.
When a user installs and authorizes your app on their Shopify store, an authorization token is generated and sent to your application. My guess is that you are storing this value judging by the error message you supplied in your post.
HOWEVER this value has to be exchanged for an “access token” that will allow your app to access the store permanently. You can see this exchange happen in the “finalize” view in the code. This is where we land after a successful app install and what we store in our “shop” table.
It has been a while since I’ve played with Shopify API but this should still be the way it works. Please let me know if you are experiencing further issues.