Embedded with Service Principale to realize the RLS

According to the online document, I download the demo. and I also input all the information.

when I try the "MasterUser" mode (Native App), I could use a new username as the viewer, and this username could be shown in the report. (The report only use the DAX function to show the username)

When I try the "Service Principale" mode, I couldn't use a new username as the viewer, and when the system want to get the embedded token from the API, it returns the error message as below:

"

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Does anybody have an experience with that? Or could you kindly give me an idea please?

If you have the relative experience, please kindly help me. thanks in advance.