Can I carry my KA-BAR in uniform?

You will never have a need or practical use for a fixed blade that size and shouldn't be using your part time job in uniform to justify such. If you really are in threat, buy a cheap wheel gun and a secondary. It's unlikely you're in such a threat though. Just listen to your NCOs and try to grow yourself first, frivolous issues are a waste of time and if you escalate it, you're going to make everyone look bad.

I can tell you that a full sized, fixed blade knife is very useful when used in the field. In garrison, not as much. Consult your chain of command, but expect a no answer.

PS in your MOS, keep it accessible in your vehicle.

Depends on your local laws and what the chain of command states.
If I was your supervisor, I would permit a K-bar during field training not in a garrison setting. I’ve had little patience with some of my knife enthusiast soldiers that are not concentrating on their actual soldier tasks.

Just some friendly advice, get the Post or Base S.O.P,

I would like to ask? Why are you so hell bent in carrying such a weapon. Are you paranoid? Has someone threaten you? Or is it. You like to show off to others, that you are well protected. Do yourself a favor. Take advise from an old (72) Nam Vet. Don't invite trouble where it's not needed. Put the damn thing in a safe place, and forget it.

I have an M-8 bayonet from Vietnam. It is still a potent weapon. Long/sharp blade sharpens when inserting and existing the M-10 Sheath. Sorry I didn't bring my M-14, that the M-8 bayonet fits.
The M-16's were just coming to Nam in the mid 60's. M-16 at the time were the biggest piece of garbage. Danger to all that used them.

Not sure the question is CAN you, but rather, SHOULD you. Short Answer is no. As a soldier you're an expert and a professional, simply carrying a 3 inch knife or a Gerber is being prepared for the duty day therefore professional. A Kabar is overkill and to be quite honest do you ever use it? Has there been a scenario where a regular pocket knife wouldn't work? Anyways, just my advice.