@Paula J  RE: How can I create an Average days to pay report?...I know it was possible on Desktop, why not qbo? 

That's a very good question!

Comparing QB Desktop reports to QB Online reports (and many other features) is like comparing a Rolls Royce to a Chevy.

QB Online isn't really designed for anyone with more than the most basic of needs. So, if you set your expectations very low, only then will you be satisfied.