I just thought I would pass along my experience or more correctly my "duh" moment in the hopes it may help someone else out
I had just finished building my Z390 Dark system with a 9900K and 2080ti. Powered it up and …..nothing aside from an A6 code. Nothing showing up on screen etc.
Rebooted several times, cleared CMOS, removed video card, etc.....same result.
Disconnected keyboard and mouse and rebooted....now get several beeps and the A6 error. That was different :-)
Then had my "ah-hah" moment. I checked to see which HDMI input the monitor was on and switched it. Then I see the EVGA bios screen and everything is good. Basically there was nothing wrong with the system just the monitor being switched to the wrong input.
Hope this helps someone with some frustration or at the least let them have a chuckle at my expense :-)