embed visuals using service principle

Hi,

I've created a report and I want to embed it into a SPA web application for our customers (external users). 

For testing, I've used the NodeJS code from here: https://github.com/microsoft/PowerBI-Developer-Samples

Followed this tutorial: https://docs.microsoft.com/en-us/power-bi/developer/embedded/embed-sample-for-customers?tabs=net-core

I could embed my report using the master user authentication but I couldn't with using the service principle.

Here is what I have done so far:

1. Created a service principle 
2. Service principle has added to a security group
3. Created an Azure app with API permissions (everything from Delegated permissions) and Client secret
4. Enabled the service principle in Power BI at the admin portal.
5. Data gateway created
6. I have a workspace and I've added the service principal and the security group as admin
7. Configured the NodeJS sample from the link above (I didn't change the code just set the configuration)

But I still getting this error message

Error Details:

Error while retrieving report embed details

Unauthorized

As I already mentioned the master user auth mode is working properly. So, I clearly missed something for the service principle but I couldn't figure out what. 

Please let me know if you need more information.