If you receive the warning "Hubspot is missing the hull_segments custom attribute. Initial sync with Hubspot may not have been completed yet. If this warning persists please contact your Hull Support." here is the likely cause:
In most cases, you see this warning because the initial outgoing synchronization from Hull to Hubspot hasn't been completed. This is likely because you haven't synchronized any users to Hubspot.
If you don't have an entry in the User Filter, no data will be synchronized and hence the warning will persist. You can create a segment and add it to the filter to clear the warning.