You need to modify /usr/share/applications/harbour-situations2application.desktop to get Sonar to install.
Basically, just add this at the end of the file to disable the sandboxing:
[X-Sailjail]
Sandboxing=Disabled
There is a mention of doing this in a comment for Situations within its Jolla Store listing.
I would recommend just restarting your phone after making the change, and then go ahead and install Sonar.
You’ll have to test out the functionality afterward to make sure everything you need Situations/Sonar to do still works as desired.