We have tried to give permissions to a service account to run reports as a batch process, but we have succeeded with that only when we give local admin permissions to the service account and we don't wish to do that.
And these lines are in ReportServerService log file:
runningjobs!ReportServer_0-26!4e80!09/10/2020-11:03:21:: i INFO: Adding: 1 running jobs to the database library!ReportServer_0-26!4e34!09/10/2020-11:03:32:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.AccessDeniedException: , Microsoft.ReportingServices.Diagnostics.Utilities.AccessDeniedException: The permissions granted to user 'domain\serviceaccount' are insufficient for performing this operation.;
The account has permissions to run web reports as defined in MS document.