Since the release of macOS Catalina applications have required notarization when they are downloaded outside of the App Store.
This requirement is added to each downloaded file prior to Gatekeeper checking for notarisation. Although this is designed to ensure that only trusted software runs on your Mac, it can limit the options when installing applications outside of the App Store.
To check if this requirement is present in a download, run Terminal and use the command xattr e.g.:
If the download contains the entry com.apple.quarantine and it is not notarized, it will fail Gatekeeper.
To get around this you need to remove this entry. This is easily performed in Terminal with the xattr command and the delete -d argument e.g.:
xattr -d com.apple.quarantine ~/Downloads/id3editor.x64.dmg
The application or installer will then run as if it was notarized.