Hi
In your scenario, please follow steps below to troubleshoot the issue:
1. Check if "the user performing the Power BI integration needs to be a member of the Azure AD tenant, a Reporting Services system administrator and a system administrator for the ReportServer catalog database". Also you can check other requirements listed in this article: Requirements for Power BI Integration.
2. Update the registration in Report Server Configuration Manager.
3. Check if browser has been configured to allow pop-ups from your report server site. And always run the browser as Administrator.
4. Repair report server via launch setup.exe, then choose Repair on Maintenance tab.
5. Use Fiddler to trace http traffic.
6. Check if any error message logged in Reporting Services error log (default location: %programfiles%\Microsoft SQL Server\<SQL Server Instance>\Reporting Services\LogFiles) around the time that you sign in the account in report server.
If you have any question, please feel free to ask.
Best Regards,
Qiuyun Yu