I figured out how to get the AppImage working on Debian if anyone want’s to do the same (as repository errors occured using apt method)
1st I tried launching it via terminal to see any errors shown.
I saw a ‘ffmpeg’ missing error so I installed ffmpeg with…
apt-get install ffmpeg
Then it worked launching from terminal…
sudo ./balenaEtcher-1.5.101-x64.AppImage
Next I needed to add the AppImage to pkexec list of allowed apps…
like so…
Open or create the file (as root)
/usr/share/polkit-1/actions/org.freedesktop.policykit.pkexec.policy
Add the following text…
(If policy file is not empty only add the text from <action id=…> to </action>)
<?xml version=“1.0” encoding=“UTF-8”?>
<!DOCTYPE policyconfig PUBLIC
“-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN”
“http://www.freedesktop.org/standards/PolicyKit/1/policyconfig.dtd”>
<policyconfig><action id=“org.freedesktop.policykit.pkexec.run-balenaEtcher-1.5.101-x64.AppImage”>
<description>Balena Etcher</description>
<message>Etcher requires SuperUser access</message>
<defaults>
<allow_any>no</allow_any>
<allow_inactive>no</allow_inactive>
<allow_active>auth_admin_keep</allow_active>
</defaults>
<annotate key=“org.freedesktop.policykit.exec.path”>/path/balenaEtcher-1.5.101-x64.AppImage</annotate>
<annotate key=“org.freedesktop.policykit.exec.allow_gui”>TRUE</annotate>
</action></policyconfig>
Change the line…
<annotate key=“org.freedesktop.policykit.exec.path”>/path/balenaEtcher-1.5.101-x64.AppImage</annotate>
setting /path/balenaEtcher-1.5.101-x64.AppImage to the correct path to where your AppImage is.
Now save the policy config file.
Now you can create a launcher in the menu’s or on the desktop or anywhere.
Set the launchers command to the path of the AppImage and prefix with pkexec command…
Eg.
pkexec /path/to/prog/balenaEtcher-1.5.101-x64.AppImage
I hope this helps some 
Bruce