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I have installed Power BI Report Server (March 2018) as an upgrade at two different installations.
One went well - on the other the service could not start - "Access denied" at every start-attempt (also as "As Administrator)
Not enought information to diagnose the problem. Suggest you run through all the configuration steps in the report server configruation tool, including checking your service accounts, DB connections and URL reservations. You can also look at the server log files, for more detailed information.