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:

1. make a right mouse click on the engine on Finder level and choose
2. Open. This opens a Finder window. Click on
3. Open again to start the application.

This problem has been solved with just:in v3 and higher.