ideally both sender and approver should have there own email address, but it is not compulsory as system send a notification to id which is updated on email id column against user id in approval user setup.
Some of possible root cause are-
1,Check Email Setup. should setup with 365 account or SMTP
2. Check all email id's are valid email id.
3. check notification setup. Notification type should be approval and method should be email. also check notification Schedule and setup instantly if you are expecting email instant
4. Check "Notification Dispatcher" codeunit in job queue.
Regards,
Alok Kulshreshtha
Please Verify my answer if you find my post helpful.
