Following.  

Let me know if you receive a response.

I too am looking for an answer to this--for now I've attached a PDF:(

Im also having the same issue. 

I have the same issue.

Same issue. It also states that Federal return and federal sch EIC must be attached also. Are you attaching those as PDF's? 

I have the same message.  I have attached the PDF but am still getting the same error.

I downloaded the MO-WFTC form directly from the Missouri website. I then completed it and saved as a PDF. Then I attached that form to the Missouri file. The one that is in our program is not the one that the state requires.

Were you able to e-file then ?

Yes, however, it took a couple of tries because it detached the form. I had to make sure it was saved in the correct folder.