In certain circumstances, such as when an installer is damaged, you'll see an alert that says the application can't be verified and may have been corrupted or tampered with during downloading. Or it says that the package was signed with a certificate that has expired and may not be authentic.

Delete the installer. If it's a macOS installer, it may be in your Applications folder. Check the date and time on your Mac to make sure they're set correctly. Download a new copy of the installer. If the new installer is also affected, please contact the maker of the software for help.