JWT is just the standard that PowerBI uses to generate a token that can be sent (you can paste your token on that site and see what is inside).
You could manually generate the JWT, but in your case the CLI tool already takes care of all that stuff and it should give you a valid token.
Double check all the parameters?
- tokenType should be 'embed' I think (hard to find any documentation on this...).
- collection is the workspace collection name found in Azure portal.
- accessKey is the secret key of your workspace collection also found in Azure portal.
- workspaceId is the ID of the workspace that you created using the 'create-workspaces' command.
- reportId is the ID of the report you uploaded.
Tokens do have an expiration date, maybe try generating a new one?
That's all I can think of...