Sometimes you can get “VT-x is not available. (VERR_VMX_NO_VMX)” error if you are trying to start x64 bit virtual machine in VirtualBox.

Failed to open a session for the virtual machine Windows 10 TP.

VT-x is not available. (VERR_VMX_NO_VMX).

Result Code: E_FAIL (0x80004005)
Component: Console
Interface: IConsole {8ab7c520-2442-4b66-8d74-4ff1e195d2b6}

There are three most common reasons for this error:

• Your CPU doesn’t support VT-x or AMD-V virtualization
• VT-x or AMD-V is not enabled in BIOS (UEFI)
• You have Hyper-V virtualization enabled in Windows

You can fix first one only by replacing CPU with a new one, but it is easy to fix second and third reasons.

Solution 1: Enable VT-x in BIOS

• Restart your computer
• Load into BIOS (press Del, F2, Esc key. Depends on motherboard)
• Find Virtualization setting and enabled it. It might look different in you system, but here are some examples: 

Solution 2: Disable Hyper-V virtualization

• Run cmd.exe as Administrator
• Execute:

  1	dism.exe /Online /Disable-Feature:Microsoft-Hyper-V

• Reboot computer

Another way:

• Open Control Panel
• Go to Program section -> Turn Windows features on or off
• Disable Hyper-V 
• Reboot computer