If you purchased the HL15.20 which comes with the new ASROCK ROMED8-2T/BCM motherboard and purchased the 2-Slot M.2 NVMe Carrier Card (Supermicro AOC-SLG3-2M2-O) once you have installed your NVMe drives and inserted the card into an open PCIe slot it will not be recognized by your system until you make the following change in BIOS:
Navigate to Advanced > Chipset Configuration > AMD PCIE Link Width > For your specific PCIe slot where the card is installed set x4x4x4x4, then Exit & Save changes and reboot.
Just posting this in case it helps someone else.