If you are simply looking for the hierarchy under the namespace inbox, you can simply do this:
{{namespace inbox}}
For use of the build in tag system the syntax is actually tags:: inbox
Though that is not necessary if you are simply looking for the hierarchy of the inbox namespace.
I could go into why your query doesn’t give results, but that has to do with how you have used your page properties as mentioned.
If you are looking for something more complex than the namespace hierarchy, let me know and we’ll work out a query for your situation.
(Or maybe there’s a useful plugin, idk)