Comment 2 for bug 1744446

Follow that howto, and remember to use not only 'primary' partitions as seen into the ubiquity output: 'primary' cant be more than 4, so set an 'extended' partition and max 3 'primary ones.

https://www.tecmint.com/install-ubuntu-16-04-alongside-with-windows-10-or-8-in-dual-boot/

As you have not followed the basic partitionning rules, that report is a false positive, and so been closed.