Why do the following 2 PCI slots not accept any graphic card risers? See image:

https://image.prntscr.com/image/nrnxtUS-RHqLOLzW6Uq5bw.png

When I plug a riser in them I can't boot. None of the other GPU ventilators spin.

this is an ASUS PRIME B250 PLUS motherboard. These 2 slots are simply labeled PCI1 and PCI2. Does this mean I cannot use graphic cards in these slots?

Bonus question: the motherboard also has 2 M2 slots. Could I potentially purchase M2 to PCIe adapters and power another 2 cards through them?
thanks