Okay, so Stackdriver has two types of project:
- Scoping Project - project for which you can assign other projects
- Monitored Projects - projects monitored by alerting policies configured at Scoping Project.
Be sure, that monitoring.googleapis.com has been enabled on scoping project. Once enabled, execute following command
gcloud beta monitoring metrics-scopes create projects/MONITORED_PROJECT_ID --project=SCOPING_PROJECT_ID
Or try to add projects via UI. Should work.