I havent found DFU device in My PC

although you turn the dfu mode in the VNM device but you havent seen it in "Universal Serial Bus Controller", "Other Device",..

1. Open Windown Power Shell 

Type command

Get-PnpDevice -PresentOnly | Where-Object { $_.InstanceId -match '^USB' }

Here you will see all usb device. now find "STM32 BOOTLOADER", "STM32 in DFU MODE", "DFU in FS Mode" => the PC already find VNM device in DFU mode, you have to find it in device manager.

Need install driver if it shows "STM32 BOOTLOADER".  (download driver for dfu here)