Dear Maintainer,

   * What led up to the situation?
     When starting octave with "octave --gui" an x window opens with the text:
      Could not setup the data required for the documentation viewer.
      Maybe the Qt SQlite module is missing?
      Only help text in the Command Window will be available.
   * What exactly did you do (or not do) that was effective (or ineffective)?
     I made sure that octave-doc was installed and installed a few extra packages: libqt5sql5-sqlite, sqlite3, sqlitebrowser and tried again.
   * What was the outcome of this action?
     That did not help. I suspect this is a build dependency problem.

I had the same problem and was able to resolve it by
installing libqt6sql6-sqlite

This should probably be added as dependency, or at bare minimum as
Recommends (but probably as Depends since otherwise you'll get that alert
every time you start octave)