All PCIe slots are backwards compatible, they just operate in slower speeds.
Your issue is not if you can install it.
You have legacy BIOS that does not support newer GPUs, I think the last series that supported legacy BIOS is 700 from Nvidia.
Another issue is that, whatever CPU and RAM you have in that system, are way too old and slow to properly use any decent GPU. Ofcourse you should forget gaming on that system as well. Only if it's games published before 2000.
Last issue I can see is your PSU. If it's same age like the rest of the system, you risk everything by adding a GPU to the equation. That needs replacement as well if you still consider buying a GPU for that system.
I would suggest that you save and buy a whole new PC as yours is WAY outdated. Even a second hand prebuilt from 5 years ago (which I DO NOT suggest you buy) would be blazing fast compared to yours.