For reasons unknown I have experienced an unclean installation of the Debian 10 TemplateVM under Qubes 4.0.3. Symptoms include:

• The only menu entry for debian-10 is Start
• The TemplateVM and derivative AppVMs does not start

It is easy to re-install the template; from dom0:
sudo dnf qubes-dom0-update --enablerepo=qubes-templates-itl --action=reinstall qubes-template-debian-10
It should be noted that an alternative minimal installation template is available which can be installed thus:
sudo dnf qubes-dom0-update --enablerepo=qubes-templates-itl qubes-template-debian-10-minimal
Due to the large size of these packages it is probably worth clearing the package cache post-installation:
sudo dnf clean packages