This error typically presents itself on an upgrade to 10.15 (Catalina) or 10.16 (Big Sur). Because the software was previously installed and working, code signing issues like this one can come up on the upgrade.
The next release of MSG Viewer for Outlook will address these issues, but in the meantime, you can download this utility and run it. It generally takes about 2 minutes. It is designed as an installer, for ease, but nothing is actually installed - and you can remove it afterwards. It just corrects this issue that occurs after upgrading from a prior version of macOS.
For any questions or further support, please submit a ticket.