Zammad Post-Installer Script returned Error Code 126

We can’t really do much about this issue.
On a “normal” system /tmp belongs to root. Maybe check that as well.

I believe that other installation processes temporary using /tmp should fail as well.
Are you able to simply create files into said directory? touch /tmp/my-test

If that fails as well, this may be an indicator.
I can’t reproduce this so it doesn’t seem to be application specific.