It will have to be reset by your main dealer,i take it you disconnected the
small plugs under the seats.

You started the car while the seats were out, didn't you? (or you turned the
key on to listen to the radio)

The Airbag is a light that demands service to correct any supposed problem,
so you need a mechanic that has the diagnostic tool to reset the light for
you. The circuit detects a fault, and the assumption is that any fault is
serious and requires tools and equipment beyond what you will have, and
therefore the circuit will not reset itself even when the fault goes away.

PS
If you watch closely, you should see the light go out after the self test
period when the key is first switched on, but then the light will
immediately come back on. It should go out for a half-second, or so, then
come on again. It looks like the light is "always on", but there is a subtle
difference between always on and going out then coming on again.