Okay, that board bifurcates bandwidth from the GPU slot to the two secondary M.2 sockets. You'll see this on
pg. 20 of the manual. This means the GPU will run at x8 (which generally isn't a problem). The reason it does this is so that you can use two Gen4 SSDs in those sockets (requires CPU lanes, not over B550 chipset).
So what you have to do is go to Settings menu in BIOS and under IO Ports you will see the
PCIEX16 Bifurcation option - set this it 1x8/2x4. That will enable both M.2 sockets at x4 (each) and the GPU at x8. You shouldn't have to do this, the board should automatically detect, but it's a new board and BIOS so who knows. That would be my workaround.
If you're freaking out about a GPU at x8, keep in mind that of existing GPUs only the 2080 Ti is at all impacted performance-wise and not by a lot, and upcoming GPUs (next gen or two) will not be bandwidth-strapped by x8 PCIe 4.0.
If you want to keep the GPU at x16 you need to get a M.2-to-PCIe adapter (NVMe, not SATA) and put it in the
PCIEX4_1 slot. If you use PCIEX4_2, you will lose some SATA ports.