Why my oil light doesn't come on when my oil is low?

Could be time for a new van.
The oil light is warning the driver engine oil pressure has dropped below 10 psi, not necessarily that the engine is low on oil.
Common causes for low oil pressure is a faulty oil pump or screen, or worn engine parts.
The fact you are also adding coolant and you don't see any external leaks sounds like the engine is reaching the end of its life.

Get an Auto Electrician to trace and fix the problem. There are partial contacts in the wiring system.

Depending on tone and depth of "clicking" it could be anything from lifters to connecting rods or piston pins....

cheapest to expensive:

Replace oil filter

if that doesn't do it (which it doesn't sound like it),

Replace engine...

Because clicking/ticking/clunking noises while having a "low oil pressure" issue (most of the time) means the engine is destroying itself.

check your distributor pick up wires, make sure they are not crossing.

That's really bad ... First of all check oil level when its cold... The clanking sounds are your valves running dry. This will cause massive damage the heads. If you oil is low fill it back up & check the engine for leaks & hope there is not valve damage. If the oil was low and you filled it & you still get the clanking noise the valves are permanently damaged & the head/heads & valves will need to be replaced.. Big costs

Have you checked for any codes that the PCM has stored? If not, that should be the first thing done. Can't diagnose a Check Engine light without getting codes because that is what the light is for, to let you know the PCM has detected a fault in one or more of the systems it controls.
Go to a parts store, most will hook up a code reader and check for codes for free.

Your readiness test will allow your PC to reset & if u are going to get a check engine light that's going to come that's the time it will come on, after the "readiness road test" Take it out & dive high & low speeds. If that light is to come on it will.

remove the dummy plug on the oil switch at the unused terminal

Two possibilities, either you have a bad oil sending switch or you may have an oil pump that has worn out. I would try to replace the oil pressure sensor first, it's the easiest and cheapest.

No, it goes out when the oil level is read to be correct. If you have the right amount of oil in the car, your pressure sensor is bad or you have oil sludge blocking the pickup tube at the oil pump. Run some Seafoam in the oil for 100 miles and then change the oil and filter (or if the pressure light stays on, add half a can of Seafoam and idle it for 20 minutes before changing the oil). Repeat 2-3 times and see if it goes away. If not, change the pressure sensor (on the oil cooler - the aluminum block the filter screws onto). If this still isn't a fix, you need a desludging kit from www.ecstuning.com. If THAT doesn't fix it, drop the oil pan and change the pickup tube and filter screen, clean out the pan, and repeat the Seafoam treatment.