There are numerous things that can cause this in the system:
A field on the report being scheduled no longer exists. Try and run the report from the Reports module and see if it errors
Every user on the schedule needs permissions to the Report Schedule module. You can grant this via Role Permissions
You have selected both CSV and PDF as the file types, please try one or the other and try again
If neither of these scenarios resolve your issue or if you have recently upgraded to version 12, please see the following Get support in Access CRM and reference the title of this article.