Seatbelt light doesn't go off - 2002 Hyundai Getz

The airbag light is on because there is something wrong with the seatbelt system each seatbelt has a electric connection and a sensor built in

There must be a wire connection to the seatbelt you left off? Some switch at the reel, some on the stork / catch end

Sorry about this.
The only way to know if this is covered under warranty is for you to confirm whether your warranty period is still active or not.
Warranty period active; take your car back to the dealer and get the seatbelt recoil fixed.
Warranty period inactive: you have to bear the cost to get the seatbelt recoil fixed orrepaired.

Just because the headlight work doesn't mean the battery is good enough to crank the engine, that takes 150"s of amps or more. Bottom line have the battery tested by a local parts store, it is free.

check all your fuses and bulbs

Unfortunately there are several causes for this, most likely a bad wiring connection under one of the front seats but it will require a diagnostic scanner to correctly diagnose the issue. You can check the connectors first, unplug them and make sure they are clean then plug them back in. If that doesn't work then it will need to be checked by an auto electrician.

check your seat belt stork , check wiring to stork ,

As far as I know that seat belt warning chime and light is there for your safety and protection, it should not be bypassed in any way.
There is no by pass seqwuence or code to eliminate this safety device.
Buckle up, its the law.

check airbag connects and have code cleared should go out

sensor in the seat belt bukle needs to be replaced