With the RX6600XT installed enter BIOS and see if the PCIe slot recognizes and shows something installed in the same slot as your GPU Card.
Do you have the PSU PCIe GPU power cable connected to your GPU card? If it is off the GPU card won't boot up.
Are the Fans running when you first boot up?
Try installing the GPU Card in a different PCIe x 16 slot on your motherboard and see if it is recognized.
In BIOS can you set the GPU card as your main Display Adapter and not the Integrated Graphics?
Normally when you install a separate GPU card in BIOS, the Integrated Graphics is disabled and the GPU card is made your main Display adapter.
Best way to know if your motherboard or GPU card is the problem try installing the GPU card in a different computer and see if it is recognized and boots up.