Power BI - Can not save report to server

i'm just download the power bi report server and power BI desktop (report server )
i could enter the web portal of Power BI report Server
then  i created the report to save to report server
but this error appeared:

then i tried to save the report to local file and upload the file to report server
but  another error says  have an error and try later...


So one of my techs made some progress and got Power BI Reports working on PBIRS for now.  The service account used in RS Configuration Manager was not a local administrator on the server.  As soon as he replaced the service account we had in the past used with SSRS with a user that is a local administrator on the server, we were able to get PBI Reports working on PBIRS.
The tech's hypothesis was that it has something to do with URL reservations, but I'm not 100% sure that's right since everything else about PBIRS was working except PBI Reports.  In any case, changing the service account made it work.

I will say we're not willing at this point to allow the PBIRS service account to be a local administrator on a permanent basis.  We're going to experiment with making the regular service account a local administrator, getting things set up, then removing local administrator rights from that user to see if things will continue to work in that manner.  If that fails, then unless Microsoft gives us an unambiguous notice that the service account MUST be a local administrator, we don't see a reasonable way to move forward.