Report Server Error scheduled refresh and Access Database

I have no idea what steps you've done to setup your scheduled refresh and no idea if you have done any troubleshooting so let's start at the beggining.

Please make sure you followed these steps:

https://docs.microsoft.com/en-us/power-bi/report-server/configure-scheduled-refresh

If you performed that, then comes the troubleshooting - Something, somewhere doesn't have permissions.  I suggest started with the individual data sources and verifying whatever user is making the auth attempt can in fact access the data source.

Also know that whatever user is being used for the auth attemp that they have access to any functions, views, procs, and of course tables that you are utilizing.

Good luck.