Is you CPU very old? It might not support virtualization
I am not an expert but wanted to chime in while others may be unavailable because it is the weekend. There are programs out there to verify if you CPU supports virtualization. Research led me to "SecurAble’ which is free, but I am not able to confirm it is open source. There are likely other tools that could achieve this
Another possibility is RAM allocation is not sufficient
Lastly if there is no bios setting, perhaps some security application installed on your host OS could be preventing it
These are all my thoughts, but someone more knowledgable will hopefully chime in with a more straightforward answer.