Like the others said, double check your connections. It’s easy to accidentally not plug something in all the way. Also, an extra preinstalled standoff wouldn’t be out of the question and I have had that happen, would be worth checking out as well. You didn’t mention what power supply you are using. New/used? Older used? Can you test the power supply on another computer to help rule that out?
Got curious and looked it up. 2x64GB DDR5 kits exist though it looks like they’re all ECC (probably a good thing in OPs use case). The cheapest kits, which are 4800Mhz CL40 sticks, start around $500 after tax.