According to your error message, I found some troubleshooting ideas provided by other users on the forum, you can see if you can solve your problem according to these steps.
1 - Open the report(s) in PBI Desktop and clear the cache (file...options and settings... options)
2 - Refresh the data in the desktop and save.
3 - Open PBI Online and remove the associated dataset(s). Note - you'll need to remove the reports from any content packs before you can remove the reports and data from the online service. Also you might want to make a note of the (failed) refresh schedule you had before doing this so you can recreate it exactly.
4 - Hop back to PBI Desktop and publish the report from PBI Desktop to Online
5 - Hop back to PBI Online and set the refresh schedule back up.