I am not trying to use SLI or Crossfire, Speedstep.
My question was and is why the primary PCIe x16 blue slot does not work in my system in a single graphics card configuration, but I do not want to spend more time experimenting, as long as the blue slot works fine when used in two graphics cards configuration (when another graphics card is installed in the black slot).
I can return one of the graphics cards, or leave both running to use for crypto-currency mining.
It depends on the purpose of your system. The cryptocurrency mining software for instance does not support SLI either, but it can use for mining as many GPUs as you have installed, even mixed AMD and Nvidia GPUs.