To clone this project, use
git clone https://github.com/tezz-io/iMessageClone.git
Go to X-Code > File > Open, Navigate to the iMessageClone folder generated on your system after cloning.
Click on the iMsg.xcodeproj inside the iMessageClone folder.
In the Project Navigator, Go to iMsg > iMsgApp.swift.
By any chance if you are not seeing any preview, use [⌥ + ⌘ + P] to refresh the canvas.
If you are not seeing the canvas itself, make sure you are on Xcode 11.0+ version and on macOS Catalina.
You can also try [⌥ + ⌘ + ↩] to get canvas if your XCode and macOS are on supported versions.
Now you will be able to preview the project!