Just had a similar issue. I'm very very new and just built my first rig. Spent 3 hours trying to get 6th card working with BOD, windows 10 x64, MSI gaming 5.

However, I read that primary display card should use full cable which I don't have, so thought to enable onboard graphics and use that.
Wallah!!  All working, 6 cards now showing up without error.