kontakt8 won't work with my Nektar Impact Lx61+ Midi controller

i can see my Midi controller(under midi) but I cannot assign the port

Just to be sure , did you click at the OFF to configure ? , reference : Configuring your MIDI Keyboard / Controller in Kontakt

Are you trying to use the keyboard on two programs at the same time ?

Windows most likely won't allow two independent programs to share a MIDI port. To test this, try closing your DAW before you assign the MIDI in your Kontakt standalone.

But here's the catch. As soon as I open cubase, and open kontakt from within cubase it again doesn't work. I suppose it's because again the midi ports are all assigned to cubase.

Please explain what you mean by : "As soon as I open cubase, and open kontakt from within cubase it again doesn't work." ? are there no midi activity in the DAW ? Or are you referring to no sound ?

If it is a sound problem then try using the Steinberg built-in ASIO Driver: information & download for both Kontatkt and DAW (Kontakt must be setup as standalone) and then Disable Exclusive Mode in Windows SoundSettings

Sorry , I do not know these products or what settings are needed but you can try using that ASIO driver for Kontakt and DAW and then maybe Disable Exclusive Mode in Windows Sound Settings ?

I am not sure about any of this.

But I think that you first need to establish if the midi works in the DAW (if you can record Midi using keyboard , DAW and Kontakt and if the problem is a Midi settings or a sound settings problem , you still did not answer that question).

Sometimes with sound problems the people can route their way out of the problem using the "VOICEMEETER BANANA Advanced Mixer" (it's free/Donationware)

But please then at least make a Windows restore point before digging too deep into changing Windows audio settings or even better take a backup.

Great , but you probably need also use same sound driver for both DAW and Kontakt.