It is normal that the device won’t accept fastboot command, as it is not in fastboot (BootLoader) mode.
So, you should issue a adb reboot bootloader before issuing any fastboot command.
If the device won’t go to fastboot (BootLoader) mode, I strongly advise to reset to stock ROM.
Please be aware that drivers a quite old, so they may not work on a Windows 10 computer (I ran them successfully on a 32b Windows 7 computer, please see here for details).