If no screen, you check if you have plug in the extra 2 molex and 1 sata needed to power the board.

In picture, i dont see plug in the pcie_pwr1 molex cable yet. check pcie_pwr2 and sata_pow1.

disconnect the pcie riser and gpu.

start up and see if boot screen appear and press F2

there is setting use onboard graphic, pcie or both.

You must set both when there are riser gpu in use.