If you  are using vector drawable use this extension:
fun  Context.bitmapDescriptorFromVector(vectorResId:Int): BitmapDescriptor {
    val vectorDrawable = ContextCompat.getDrawable(this, vectorResId)
    vectorDrawable!!.setBounds(0, 0, vectorDrawable.intrinsicWidth, vectorDrawable.intrinsicHeight)
    val bitmap = Bitmap.createBitmap(vectorDrawable.intrinsicWidth, vectorDrawable.intrinsicHeight, Bitmap.Config.ARGB_8888)
    vectorDrawable.draw(Canvas(bitmap))
    return BitmapDescriptorFactory.fromBitmap(bitmap)
}
And set to the marker icon:
val markerOptions = MarkerOptions().position(latLng)
         .title("Current Location")
         .snippet("Thinking of finding some thing...")
         .icon(bitmapDescriptorFromVector("Your icon resource id"))// call extension here
mMarker = googleMap.addMarker(markerOptions)