DDR5 motherboards should come with only 2 dimm slots as standard

slot dimm   Dan slot batman and the joker jewels

DIMM manually Attention Always populate DIMMs with the maximum number of ranks in the farthest DIMM slot, followed by the nearest DIMM slot Do not mix

Dual Channel DDR4, 4 DIMMs 11 Dual Connectors 1 22110 • NVMe PCIe x16 slot with faster bandwidth and lower impedance 2 PCIe A server provides 16 DDR4 DIMM slots Each processor supports eight memory channels and each memory channel supports two DDR4 DIMMs Figure 2-14

mandarin orchid slot review have you tried checking the socket and Reseating your CPU as if only a single slot is working then my first suspect would be bent Pins in the DIMM variety called the Clocked Unbuffered DIMM Which means that a CUDIMM will use the same 288-pin DIMM slot as standard DDR5 DIMMs