The reason this error message comes up is because the report is a component of a dashboard. You can't delete reports if they are associated to a dashboard component.
To find the Dashboard Component associated to the report follow these steps.
A. Create a custom report type:
- Go to Setup | Create | Report Types | New custom report types.
- Set the primary object to Reports and then the secondary object will be Dashboard components. Also set the Dashboard components to 'A' records may or may not have related 'B' Records.
- Run report using that report type and include the following fields:
- Report Name
- Dashboard Title
- Dashboard Component ID
- Created By: Full Name
You may add a filter Report Name equal to "Add name of Report" to limit the result.
B. Delete the Dashboard Component
- Copy the Dashboard Component ID and paste it on your browser URL bar, right after your instance name. For example:
- Dashboard Component ID: 000XXXXXXXXXXXX
- Browser URL: https://na5.salesforce.com/ 000XXXXXXXXXX
- If you can open the Dashboard Component, simply delete it.
- If you can't open the Dashboard Component due to a Data Not Available error message, then this means that this Dashboard Component is saved on someone else's Personal Dashboard Folder.
- Have the user whose personal folder where the dashboard is stored delete the Dashboard Component ID, as explained in Step 1.