Experience Cloud Site Users receive an 'Insufficient Privileges' error message within Reports

An external user, such as a user with a Partner Community or Customer Community Plus license, may receive an "Insufficient Privileges" error message when viewing reports with certain Standard Report Types, such as Accounts with Assets, or Products, even though they have access to the folder and can view other reports in that folder. For Custom Report Types, ‌reports open but are missing expected data.

For external users to have access to a report, the Org-Wide Defaults (OWD) of each object included in the report type must be set to Private, and access must be granted via sharing rules. To address the error, change the object's Default External Access (If External sharing model is enabled) else change Org wide sharing setting to Private under

1. Navigate to Setup | Security Controls | Sharing Settings.
2. Then extend the access to Partner User through Sharing rules if necessary.

However, some objects don’t have org-wide default settings, and can’t be shared via sharing rules. These objects aren’t visible to external users in reports. 

One example of such an object is Products. Products is a standard object included in many standard report types, most notably "Accounts with Assets" and "Products" report types. Since Products do not have org-wide default settings, it is impossible to set these to Private. Another example involves Activity CRT. The underlying object of Name is not available for sharing settings. Since there is a lookup between Activity and Name, fields that are added through the Name entity are missing in reports.

More details on how to create a Custom Report Type here

Links to Related and Supporting Materials:

Idea Exchange Article

• Allow us to share a price book report to the PRM partner portal
  
  Note:
  If you are able to set the object’s Org-Wide Default (OWD) to Private and create a sharing rule, but the rule only allows sharing with Site Guest Users (and not authenticated Partner or Customer users), this indicates that the object does not support external sharing for authenticated Experience Cloud users.
  In such cases, the object cannot be exposed in reports to those users, and reports including that object may return an “Insufficient Privileges” error.