I keep getting a stop engine low oil pressure warning on my 1996 Cadillac Sedan Deville . I just changed the oil and filter.Do I need to replace the oil sending unit and where is the oil sending unit located .

It could be oil pressure is low, in which case shut off engine and haul to shop. Or it could be a false indicator, i.e. faulty oil pressure sense circuit (sending unit/cable/connect).

I see more than one issue in your story. Its possible the computer lost memory and will need to relearn. But that would not explain the knocking noise. The oil pressure warning might explain the noise if oil pressure really is low. Running without the belt for even 5 min would not cause the problems you describe.
I would focus on the low oil pressure warning and the noise first.

Make sure oil level is full, possible faulty oil pressure sender. replace sender and recheck, if warning still comes up, possible internal engine issue.

Replace the oil filter as it is clogged very cheap fix. I hope this helps.

Possible faulty oil pressure sending unit telling the computer that the pressure is low. Recommend replacing sending unit and see if that solves it.

The oil pressure sending unit is behind the front engine mount. You will need a deep special oil pressure socket to remove and install.It is NOT an easy task. There is no short cut but you might find it easier to remove the oil filter adapter, if you do that you should replace the "O" rings on the adapter. The oil pressure sender is mounted on the filter adapter

do you have enough oil??????? if so, it is probably the oil pump.

If the a/c does not engage as a result of the electronic control system in your 1996 Cadillac, the climate control system may need to be replaced.

However, if the problem is being caused by low charge of R134a in the system, have the system charged before changing that electronic part.

I have never seen a Cadillac Climate Control System do this, which leads me to believe that it may be more electronic than refrigerant pressure related.

If the refrigerant pressure was low in the system, the climate control system should still stay on 'auto' mode, I think, and simply blow warm air.

always start at the least expensive item. ths would be the oil filter. if its a double wall one,switch to a single. if this doesnt work,then test the oil sending unit. alot of the older caddys do this,and as long as you dont hear lifters tapping,i wouldnt sweat it.

The idle may be to low or the motor is just old and a new set of rod bearings is need with an oilpump to bring the pressure back up.