Please check your tutorial, have some bug never fix.
https://forge-tutorials.autodesk.io/tutorials
This one is next section for : https://github.com/chuongmep/svf-viewer
Please check end repo to compare result.
Setting file .env :
FORGE_CLIENT_ID=<your client id>
FORGE_CLIENT_SECRET=<your client secret>
FORGE_CALLBACK_URL="http://localhost:8080/api/auth/callback"
SERVER_SESSION_SECRET="custom-encryption-key"
command :
npm install
npm run start
Or run by command :
node server.jsLink : http://localhost:8080