Original Post: https://gitlab.com/fhidalgo.dev/install-forticlient-on-debian-11-bullseye/-/blob/en/README.md
I am just adding this here for my sake of retrieving easily.
In Debian 11 bullseye there is a package that has been removed called libappindicator1. This is used to display notifications. However, there are still many packages that have it as a dependency, such as the Forticlient VPN client.
To resolve that dependency, all you have to do is mock the package with equivs: