Failed to open a session for the virtual machine Kali-Linux-2019.1-vbox-amd64 .
The native API dll was not found (C:\Windows\system32\WinHvPlatform.dll) (VERR_NEM_NOT_AVAILABLE).
VT-x is not available (VERR_VMX_NO_VMX).
There is a bug in VirtualBox, it is already documented and recorded under Ticket #18387.
It has to do with Vbox failing to properly detect and use Hyper-V under Win10.
There is a longer discussion and some possible solutions discussed on the VirtualBox forum https://forums.virtualbox.org/viewtopic.php?f=6&t=90853
One solution is to open command prompt as administrator and run
dism.exe /Online /Disable-Feature:Microsoft-Hyper-V
After that reboot try VirtualBox again.
thanks Tiron, i had been testing some labs for azure which had been relying on hyper v then finding all my other Vbox labs not functioning.