1996 Chevy Lumina Coolant Leaking into Engine

Check your DTC(s).
Check for external leak when car is hot (it'll be easier to find a leak with system under pressure): water pump, hoses, heater core, thermostat housing, radiator, overflow tank, etc.
Check carefully your intake manifold gasket for leaks as well. It's pretty common on your model.

If no leak found, you may have an internal leak ( leaky/blown headgasket, cracked head).
Do you have a lot of white smoke coming out of your tailpipe?
Have you checked your oil? Does it look abnormal, is your level of oil high?
Does your car crank for an unusually long time when you start it?
If you suspect a head gasket, do a compression test on each cylinder.

Also, you could have a leak in your ATF cooler. Do you have brown spots in your coolant?