I had this problem, understand this isn't what was wrong in the OP's case, but in my case I did have a Startup class, it just wasn't finding it by default.
My problem was the I had spaces in my Assembly Name, and hence the default namespace was different from assembly name, hence the namespace for the startup class was different than the assembly name.
As the error suggests, by convention it looks for [Assembly Name].Startup for the class... so be sure the namespace for your Startup class is the same as the Assembly name. Fixed the problem for me.