Hi Ryan,
It's because you haven't assigned an initial renderer to your layer.
let layerRenderer = new RasterStretchRenderer({
colorRamp: colorRamp,
stretchType: stretchType
});
const layer = new ImageryTileLayer({
url: "https://sampleserver6.arcgisonline.com/arcgis/rest/services/Elevation/MtBaldy_Elevation/ImageServer",
opacity: 0.9,
renderer: layerRenderer
});
Without it, there's nothing to clone in your updateRenderer function.