cannot install ubuntu 20.04 server on virtualbox

I had the same issue.
(PC: Windows 10 (20h2), CPU Athlon x250, 4gb RAM, GTX 650ti; Windows 10 features or components - default (Hyper-V isn't installed))

Settings which worked for me:

1. Settings > System > Motherboard : Chipset choose ICH9; enable "Enable I/O APIC"; enable "Hardware CLock in UTC Time".

2. Settings > Processor : 1 cpu; enable "Enable PAE/NX".

3. Settings > System > Acceleration : paravirtualization interface - Hyper-V; both boxes checked in hardware virtualization. (have no idea why it works, cause Hyper-V isnt installed on the Windows)

All other parameters are by default.