What you need to do is go to your "System Preferences" --> then to "Security & Privacy" --> In the "General" tab you "Allow apps downloaded from:" But before you can make any changes you need to press a lock in the bottom left corner to allow changes to be made.
And if you have already tried to open raft it will be a own option that you can select.
Hope this helps. My first guide ever. Well I did explain the way to the bathroom once. But that was different.