Generate Bearer Token for React embedded report

Hello - Looking for some direction and detailed instructions, please.
I am trying to generate a token for embedded reports in a React front end.

I have followed this article: https://medium.com/globant/how-to-embed-a-power-bi-report-in-react-f27e975e0a

I found this code: https://github.com/KotanaSai21/powerbi-embedded-React-demo/blob/main/GenerateToken.csx

I wrote a Java API for React to call in case it needed to be client->server->client thing.

Step 1 (in the article) will work; I get an access_token.

However, using that access_token as a bearer token in Steps 2 and Steps3 result in a 401.

(Using Postman)

Also, when using Embed Token - Generate Token - REST API (Power BI Power BI REST APIs) | Microsoft Learn,

(as also described in Step 3 of the article), it is successful from here when I "try this", but again a 401 from Postman. I notice that the Bearer Token used here in the MS Learn link is much longer than the access_token I receive in Step 1 from Postman.  Where/How does it get the Bearer Token used in the MS Learn link?

Looking for some direction on doing this programmatically, please - either via React or Java microservice - I just need it to work. 😃
I am using a service principal for tenant id, along with client_id,  and client_secret, ect.
Thanks in advance-