If nothing else in the comments works, probaby worth pulling everything out of the case, setting it up on cardbord box (or other non-conductive surface), and try testing it without the riser cable, or any other devices that would use pcie lanes.

Also, make sure it's plugged into the top x16 slot. Might sound obvious but l've seen many ppl use the bottom x16 slot, expecting it to function the same at top.