Due to a security feature in Windows 10, Oracle's VirtualBox will now blue screen a Windows 10 PC whenever you try start a VM in VirtualBox. The bluescreen error is vt-x is not available.
There are two possible ways to resolve the error until Microsoft comes up with a permanent fix.
Install the Device Guard and Credential Guard hardware readiness tool available from Microsoft at: Device Guard and Credential Guard hardware readiness tool. However, this fixs only works for a month until the monthly security patches are released in which case you have to run the tool again.
The other option is to uninstall the Hyper-V Windows Feature from the Control Panel.
The VirtualBox team is aware of the problem and has a ticket open for it Ticket 16801 However, the ticket has been open since May 2018 and the general concensus is that Windows can only have one Hypervisor running at a time and the problem should be resolved by Microsoft and not Oracle/VirtualBox team.