If The Unarchiver isn't working - try this:
Right click the app and look for the word copy but don’t click it yet. Hold the option key and it should say “Copy Spacecorpsxxx as pathname”. Open your terminal and type “sudo chmod -R 755”. After that use the paste command so it can paste the pathname into the terminal. It should then ask you for the password for your Mac to allow the action. It should be able to open after all that.