Power BI Report Server unable to Refresh Reports

To "see" the underlying error messages for the failures you can use something like the query below (this should be run against the ReportServer DB of your PBI SSRS Server)

SELECT
	sj.name AS SQLAgentJobName
	,c.name AS ReportName
	,c.[path] AS ReportPath
	,s.[Description] as SubscriptionName
	, rs.SubscriptionID
	, s.laststatus
	, s.eventtype
	, s.LastRunTime
	, sj.date_created
	, sj.date_modified
	,err.[Message] AS StatusMessage
	,err.SessionID
	,err.Errs
	,ErrData.ErrCode AS ErrorCode
	,ErrData.ErrMsg AS ErrorMessage
FROM ReportServer.dbo.ReportSchedule rs 
		INNER JOIN msdb.dbo.sysjobs sj
            ON rs.ScheduleID = CAST(sj.name AS uniqueidentifier)
			and 101 = sj.category_id
        INNER JOIN ReportServer.dbo.Subscriptions s
            ON rs.SubscriptionID = s.SubscriptionID
        INNER JOIN ReportServer.dbo.[Catalog] c
            ON s.report_oid = c.itemid
		LEFT OUTER JOIN (SELECT MAX(SubscriptionHistoryID) AS SubscriptionHistoryID, SubscriptionID FROM dbo.SubscriptionHistory GROUP BY SubscriptionID) sh
			ON rs.SubscriptionID = sh.SubscriptionID
			LEFT OUTER JOIN (SELECT SubscriptionHistoryID, 
									[Message],
									JSON_VALUE(Details, '$.SessionID') AS SessionID,
									JSON_QUERY(Details, '$.Errors')  AS Errs
							FROM dbo.SubscriptionHistory ) err
				ON sh.SubscriptionHistoryID = err.SubscriptionHistoryID
				CROSS APPLY OPENJSON(err.Errs) WITH( ErrCode INT '$.ErrorCode', ErrMsg NVARCHAR(4000) '$.Message') AS ErrData
-- to find specific report last status 
-- WHERE e.name = 'Usage Stats'
-- to find failed status
WHERE 
		LastStatus <> 'Completed Data Refresh'

You can obviously filter this for specific reports and or date ranges as required.

This should at least point you in the right direction

Note that there is a "magic number" in that SQL that makes it work.

INNER JOIN msdb.dbo.sysjobs sj
				ON rs.ScheduleID = CAST(sj.name AS UNIQUEIDENTIFIER)
				   AND 101 = sj.category_id

the 101 is the category_id of the Job Category called "Report Server"

you can find the correct value for this using the following.

USE msdb
GO

SELECT * FROM dbo.syscategories

I think 101 is safe on most systems but it may well be different on your installation