Hi sandrine , By seeing above error message it seems like you are deleting a report folder not the report.If you want to delete a folder you have move all the reports in that folder to other folder .
If you are trying to delete a report,click on delete button next to you report name.You can’t delete reports in others’ personal folders. You also can’t delete reports used by dashboard components or reporting snapshots unless you first delete the dashboard component or reporting snapshot.
By seeing above error message it seems like you are deleting a report folder not the report.If you want to delete a folder you have move all the reports in that folder to other folder .
If you are trying to delete a report,click on delete button next to you report name.You can’t delete reports in others’ personal folders. You also can’t delete reports used by dashboard components or reporting snapshots unless you first delete the dashboard component or reporting snapshot.
Please refer below link which might help you in this
https://help.salesforce.com/articleView?id=reports_delete.htm&type=5 (https://help.salesforce.com/articleView?id=reports_delete.htm&type=5)
Hope this helps you
Let me know if this helps you. Kindly mark it as solved so that it may help others in future.
Thanks and Regards