MACHINIST X99 Dual CPU Motherboard, Intel (5th/6th Gen) LGA 2011-V3 Computer Server Motherboards (E-ATX, PCIe 3.0, DDR4, Dual LAN, M.2, SATA 6Gb/s) for Intel Core i7, XEON E5 V3/V4 Series, X99-D8-MAX

That Amazon link shows an MACHINIST X99 (B0BFDX44L2) with a single cpu slot, perhaps you meant B0CT3HDPPQ You will have to consult the motherboard manual (or online documentation) to determine if this system can be operated in single-cpu mode, or if it requires both cpus to be installed. I would suspect single-cpu mode requires you not to have the other one installed -- try installing only one cpu, maybe also try swapping them, or even reset the CMOS. PS: An FF error code could mean either (1) the POST succeeded or (2) the cpu failed to properly come out of reset to execute the BIOS code--the last one could mean just about anything and could be issues with system power, a bad cpu, or even a damaged motherboard (broken traces or solder joints).