You can't use InvokeRepeating for coroutines. You don't need to explicitly set 1 or 0 anyway; you can just alternate between them, so the function doesn't need to be a coroutine.

// Use lowercase for variable names; it's less confusing
var beaconLight : Light;
var triggered : boolean = false;
var warningSound : AudioClip;

function Start(){
    beaconLight.intensity = 0;
    InvokeRepeating("Beacon", 0, 1);
}   

function Beacon(){
    // beaconLight is already a Light, so you don't need to refer to .light
    beaconLight.intensity = 1-beaconLight.intensity;
    // You don't need "return" at the end of functions. They stop anyway.
}