There are three common culprits for the type of error the user is seeing:
VT-x is not enabled in the BIOS
The CPU doesn't support VT-x
Hyper-V virtualization is enabled in Windows
Since the user already eliminated the first two possible culprits, the next step is to open a command prompt as administrator and run the following command:
dism.exe /Online /Disable-Feature:Microsoft-Hyper-V
Afterwards, reboot the PC and try VirtualBox again.
