macOS Catalina introduced a new feature called Gatekeeper, which is designed to keep your Mac safe from malicious apps. In order for Mac apps to run without being flagged as potentially malicious, there is a new notarization process in place for all apps distributed outside of the macOS App Store. The new notarization process will be enforced starting February 3, 2020. All currently released ArcGIS Runtime SDK for Qt installers and sample viewers have not undergone this process, so these released apps will begin to be flagged when opened. The next release of the software will be notarized. For the time being, the installer, post installer, and sample viewers can be run by going to System Preferences > Security & Privacy > under the General tab, find the app you want to run, and select Open Anyway. These details and more can be found in the Apple documentation.