I just bought a 50mm 1.2 lens and its a Ai lens. I have been told it will need to be modified to work on my camera

I have a nikon d750, so they have the same f mount. I'm aware it's a manual focus but I have been told it will need to modified to work correctly so I'm wondering how that will need to be done.

Any Nikkor AI lens will work just fine on your D750.

I do not know who told you that you needed to modify the lens, but they either did not know your lens is an AI lens or they are not familiar with old Nikkor lenses and their use on current Nikon digital SLR cameras

If you look on page 427 of your user manual, you will see which Nikkor lenses will function properly on your camera and any additional notes that apply