If permissions to the reports are granted, sharing settings to the folder are set, and it is not a matrix style report and users still see this error, it is likely that they have High assurance session required enabled in their Salesforce org under Access Policies.
What to Do
- Disable the High assurance session required policy. This is found in Salesforce Setup > Customize > Reports & Dashboards > Access Policies.
- Use Conga Queries, which are unaffected by this Access Policy.