You've restricted logins somehow that is incompatible with Report Sender.
Report Sender must be able to login as users to the Reporting API in order to get the report data. To see why the authorization failed, check the Login History on the User's record. Most commonly this fails due to IP restrictions. From Setup, go to Users, then open the User that is having issues. Under Login History, look for unsuccessful logins and make note of the error.
The most common cause is if you have restricted logins to certain IP addresses. If you have IP restrictions on a profile for a user that is using Report Sender, you'll have to add the relevant Salesforce IP ranges. View the Help article on Salesforce IP ranges and add the ones that apply to your organization, or disable Trusted IP ranges.
Many security settings affect login issues, so please contact us if you're unable to figure out the issue with the settings.