Does he mean the actual airbag, or the diagnostic module?
Ok, there are three possible faults here. I'll list them in order of likelihood.
1) The airbag is faulty. Yours might be a Takada, in which case Nissan would probably replace it for you under the worldwide recall -
click here for info, it covers your model vehicle. If your vehicle has already been recalled, go to 2, because the airbags are usually quite reliable.
2) The clock spring has failed. This provides an electrical connection from the diagnostic module to the airbag. If this connection is broken (usually the clock spring is at fault, they are a bit delicate) the diagnostic module will not see the airbag and will report that the air bag is faulty/missing.
3) The diagnostic module has failed. Mine has. It's a box under the cup holders in the centre console behind the gear lever. It has three plugs and apart from "Bosch" and "Nissan", it has "SRS Airbag" on it. Your Thai version will not fit a Spanish version and vice versa. They cost about $1,025 brand new (I checked yesterday before buying one at a wrecker).