On macOS Catalina I cannot open the downloaded installation .dmg file because it’s not signed…and I’m pretty sure you also need to notarize you app itself with your cert you obtain from Apple to get pass macOS Catalina security check…
Canyou get your app ready for Catalina now? To learn more about new macOS security, watch this: https://developer.apple.com/videos/play/wwdc2019/703/