My friend and I are college engineers, what don’t we know? (I will not promote)

A friend and I are working on a start up together and the technical side of it is going well. However, we recently met with a professor who is a big name in the university’s innovation program and he told us we weren’t ready to meet with investors and that he could tell we were engineers. He told us we either needed to learn the business side or bring on a business minded person. So my question is, what shortcomings have you all seen in the engineering mindset and what would you recommend we do (books, papers, podcasts, etc) to become better entrepreneurs.