Hi
According to your description, when you pin the report item to Power BI, it will prompt you enter Power BI credential. Once you enter the account, it will throws an error, right?
In your scenario, please use entered account to access Power BI service, verify the account is valid. Then please sign in the Power BI account in My Settings to see if it can be successful. Please refer to this article: My Settings for Power BI Integration (web portal).
In addition, as the error message doesn't contain insufficient information, please check the Reporting Services error log (default location: %programfiles%\Microsoft SQL Server\<SQL Server Instance>\Reporting Services\LogFiles) around the time that the error message thrown out. Please post the detail error log for our analysis if possible.
Reference:
Pin Reporting Services items to Power BI Dashboards
If you have any question, please feel free to ask.
Best Regards,
Qiuyun Yu