Let assume that you installed Audit Collection and gave some users Report Operators role. After they run some reports from Audit Report folders they get error
An error has occurred during report processing. (rsProccesingAborted).
Cannot create a connection to data source ‘DB_Audit’. (rsErrorOpeningConnection)
What is the problem? When you are running other reports (not AC Reports) from SCOM console they are run in context of Operations Manager Data Reader you created. Because ACS is not part of SCOM reporting role it does not share same security context. Account you use to run Audit reports will not have any rights to read from SCOM audit database. The best explanation (old one but good one) I read is written on this blog .
To solve this we need to give account access to OperationsManagerAC database. Just log in to you SQL instance and give it db_datareader access. Best practice would be to create group with name for example ACReports users, and give this group db_datareader access. All users inside group will have access to Audit reports, regardless of they SCOM Reports Operator role.
This is also very specific written on SCOM 2016 documentation link
The installation procedures for ACS Reporting do not differ, but the application of access control is different. By deploying ACS Reporting on the same SQL Server Reporting Services instance as your Operations Manager Reporting, the same role-based security applies to all reports. This means that ACS Reporting users need to be assigned to the Operations Manager Report Operator Role to access the ACS reports.
In addition to membership in the Operations Manager Reporting Role, ACS report users must also be assigned db_datareader role on the ACS database (OperationsManagerAC) to run ACS reports. This requirement is independent of the presence of Operations Manager Reporting
I hope this helped you in getting your Reports utilized, because it is very important part of SCOM software.