1998 cheverlet pickup,security light came on and

can't be reset. the security system consist of the igntion key, igntion key cylinder, wire harness and module.
the most likey cause of this is the tiny wire harness from the igntion cylinder to the module is broken where it goes into the cylinder.
you can try using another igntion key, if still persist you'll need to get this fixed, sooner or later this will not allow you to start the engine.