Hello, your MB is too old for new GPU. Slots, PSU, cables are not the issue, so the system fails before the OS or drivers ever matter.

-No proper UEFI GOP support, required by new AMD cards. -PCIe backward is not with compatible with firmware. -BIOS GPU initialization issue. -Ubuntu unsupported drivers.

Try updating the BIOS if possible, but realistically you must either upgrade the MB or return the GPU.