Bootcamp issue: The startup disk cannot be partitioned or restored to a single partition
I had the same issue. I solved it with following steps:


Stop Bootcamp Assistant 
Eject all external drives (USB sticks and
mounted ISO files) 
Restart Mac 
Relaunch Bootcamp Assistant  


Normally you will be able to follow restore the previous state without Bootcamp partition.

By the way in my case the origin of the problem was probably the use of a MSDN ISO of Windows 10 including multiple installs.

I downloaded an ISO from https://www.microsoft.com/fr-fr/software-download/windows10ISO and could complete the Bootcamp Assistant.