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the Freezer is not freezing at all.
The fan, condenser works, as it is continuously blow air there. the condenser is warm.
I am assuming that by condenser you also mean the compressor re: it is warm, you can hear that it is running and you can feel it vibrating.

I can feel the compressor is warm.( this is the component that has the dome shape which has two pipes in/out for freon, beside this one, there is a fan with many grids which I think is used to cool the freon, accumulated some lints, but not too bad as I cleaned once several months ago) but not sure it is vibrating

I thought about two possiblities
1. the motherboard is faulty
I doubt this as the compressor is running so the motherboard is controlling it and trying to lower the temperature in the freezer.

2. low on freon
Could be this OR you are totally out of freon OR the compressor is toast OR there is contamination/plug in the sealed system
Remove the cover at the back of the freezer so you can see the evaporator coils.
If there is no cooling/frost then it is probably one of the above last 3.
If there is some frost ice but it is very patch or just where the freon enters the evaporator coils then you are probably low on freon.
Any of the above will require a pro to fix it and it will be expensive.
Often it makes more sense to put the dollars into a new unit.
Check your warranty, the sealed system usually has a longer warranty.
Be sure to check exactly what is covered under warranty.
Get a written quote.
Note: It makes no sense to just add freon as it will just leak out again, so the leak must be found first.

If the coils are heavily frosted/iced over you may have a defrost problem.
If the coils have just a light coating of frost then check that the evaporator fan is running. One thing to be aware of here is that if the unit has just completed a defrost cycle the coils will be clear.

The evaporator fan is the one above the light?
Yes, inside the freezer compartment.

That always runs in the freezer, the machine has been on this for several days. Is it possilble that the thermo is faulty?, i,e, it reports the freezer is cold enough? however, if that is, then the compressor should stop.
Yes it should shut off the compressor as long as the compressor is running the unit is trying to cool down. I am not sure why it shows it is cold enough.

You have got lots of frost so things are looking up, the frost does indicate that it is actually cooling (no cooling=no frost)
If it was a sealed system problem you probably would not have a build up of frost depending on how long the unit has been like this.

You have to remove the covers in the freezer so you can see the evaporator coils and confirm whether or not it is a defrost problem.

Now it seems like a defrost problem, I removed the defrost heater( which is glass tube heater). Inside the tube, it looks black, I can barely see the meta coil( does that indicate problem?). I ohm it and find it to be infinity -- that should mean the heater is bad
You are correct. The heater sounds like it is toast. They usually measure about 20 ohms.

Now do I also need to check the thermo defrost?
Yes.

I dryed all water and measured the thermo defrost ohm to be inifinity too. --- when it is cold, it should not be infinity, am I right?
You are correct.
At room temperature it should be infinity.
When frozen/cold it should be 0 ohms.
Stick it in a neighbors or friends freezer and then check it,
Also you could just hold it onto an ice cube. Get good contact between the metal face and the ice cube and do not touch the metal face with your fingers.
One thing to note about these is that if they look bulged out replace them even it they check out OK.