MACHINIST X99 Computer Motherboards, LGA 2011-V3 (Intel Core i7 5th/6th Gen) ATX Server Motherboard with VRM Fan (PCIe 3.0, Dual M.2, SATA 6Gb/s, DDR4, Gigabit Ethernet) for Intel XEON E5 V3/V4 series

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).