The most common cause of this error is an antivirus program. These programs are designed to protect your computer from viruses and other harmful data. If you're using an antivirus, the attachments might be blocked. This is a common error and isn't hard to fix. To fix the problem, simply disable the antivirus and restart Outlook. In most cases, this will fix the problem. You should now be able to see attachments in your Outlook inbox.

See also - https://emailspedia.com/unable-to-open-outlook-attachments/