It sounds like you're trying to access that's on an organizational site on ArcGIS.com (youcompany.maps.arcgis.com) When I have to access items on my organization, I have to use OAuthInfo and IdentityManager to log in.
This is an example of the code I use to get items that aren't public
require([
"esri/Map",
"esri/views/MapView",
"esri/layers/FeatureLayer",
"esri/identity/OAuthInfo",
"esri/identity/IdentityManager"
], function (Map, MapView, FeatureLayer, OAuthInfo, identityManager) {
let layer, map, view;
const portalUrl = 'https://yourCompany.maps.arcgis.com';
const info = new OAuthInfo({
appId: "xxxxxxxxxxxxxxx", //*** Your Client ID value goes here ***//
popup: false // inline redirects don't require any additional app configuration
});
identityManager.registerOAuthInfos([info]);
// send users to arcgis.com to login
identityManager.getCredential(portalUrl);
identityManager.checkSignInStatus(portalUrl).then(function () {
layer = new FeatureLayer({
portalItem: {
id: "itemID"
},
outFields: ["*"]
});
map = new Map({
basemap: 'oceans',
layers: [layer]
});
view = new MapView({
container: "viewDiv",
map: map,
center: [-153, 63],
zoom: 4,
popup: {
autoOpenEnabled: false
}
});
});
});
