In order to run ThinLinc Server on Pop_OS! An additional profile needs to be created

$ sudo nano /opt/thinlinc/etc/conf.d/profiles.hconf

default=pop
order=unity ubuntu gnome gnome-classic kde xfce cinnamon mate lxde pop  <-------
[... rest of file ...]

[/profiles/pop]
xdg_session=pop
name=Pop
description=The Pop_OS Gnome desktop environment
screenshot=
cmdline=
testcmd=
description_from_testcmd=false

For details see:

• https://community.thinlinc.com/t/create-a-custom-executable-profile-zorin-budgie-pop-os-etc/1226/1