When you launch the engine or the interface at the very first time with macOS Sierra, then a verification process will appears, which can take a few minutes. Please wait for this process to finish and then you can use just:in. This is only necessary at the first time launching just:in.
If this fails completely, cancle the process and start it again by:
- make a right mouse click on the engine on Finder level and choose
- Open. This opens a Finder window. Click on
- Open again to start the application.
This problem has been solved with just:in v3 and higher.