Summary
When installing the MSA, you get the following error message:
Upgrade is not supported for the current installed version. Uninstall the current version before installing the Messaging Security Agent. Refer to the Administrator Guide for requirements.
When you try to uninstall the MSA from Add/Remove Programs, the uninstallation does not continue and there is no error message.
Details
To resolve the issue:
If the issue persists, refer to this article: Troubleshooting MSA installation issues.