I think you need to switch your target PCIE slot to “4x4” mode?
Under [Advanced\Onboard Devices Configuration], scroll down to PCIEX16_* Bandwidth
Select correct slot, then switch [x16 Mode] to [PCIe RAID mode]
==> [x16 Mode] . . . Run x16 mode (i.e. x16 graphics card)
==> [PCIe RAID mode] . . . Run x4+x4+x4+x4 for up to 4 PCIE devices
On page 1-7 in the motherboard manual, see:
[Hyper M.2 X16 series card configuration]
“PCIe bifurcation settings in PCIe x16 slots with AMD Ryzen Threadripper PRO Series Processors”
On page 33 in the BIOS manual, see:
[PCIEX16_1-7 Bandwidth]
For details, see this post.