I too am running a fresh (test) install of SmithCart 3.96 on DNN 5.06 and I noticed the same error.
In my case I had a Smith.CategoryMenu Module in the left pane next to my Buy It Now module. It was the Category Menu that was causing the problem in my case. I removed it, re-added it and the error message went away.
At the time I was playing around with adding a second Buy It Now module and a second Category Menu.
Maybe this will help.