While downloading Designer on Mac, sometimes quarantine attribute such as com.apple.quarantine is included in the Designer application that prevents you from launching Designer. You must clear the quarantine attributes to solve this issue. To clear the quarantine, open the terminal on Mac and execute the command:
# xattr <path where Designer.app is installed>
# spctl --assess --verbose=4 --type execute <path where Designer.app is installed>
# xattr -dr com.apple.quarantine <path where Designer.app is installed>
NOTE:You will encounter this issue on Mac for any application that has not been downloaded from Mac App Store. This is to ensure that only trusted software runs on your computer.