Report Sender accesses reports as the user that is sending the report. That's how we ensure that the report contains the correct data. To make sure that Report Sender continues to work as expected after enabling MFA, please make sure that you enable MFA via a permission set, and not at the profile level. Please see the following Salesforce help article for instructions on how to set this up:
If you have enabled it at the profile level and are having issues with reports not being sent, please add the permission set via the instructions in the article above, and then remove the requirement at the profile level.