Garmin’s Connect IQ developer instructions are here. It appears that the Linux version of their SDK Manager has not been updated in many years. When unpacking connectiq-sdk-manager-linux.zip bin/sdkmanager will be extracted. When executing this you’ll find at least two libraries that are not present on Debian 11, Bullseye. Garmin’s SDK Manager binary depends on libwebgtk-1.0 and libjpeg8. These libraries are very old and have been dropped from distributions.
It is easy to install the missing libwebkitgtk dependency by adding stretch to /etc/sources/list. The same technique will not work for libjpeg8 because it much older, last appearing in Debian 7, Wheezy. Wheezy was released in 2013! The technique used for Stretch didn’t succeed due to missing keys. I gave up trying to locate and import the keys and opted to install the binary package, downloaded from an archive, instead.
Add the following line to /etc/apt/sources.list or create a new file in /etc/apt/sources.list.d.
deb http://deb.debian.org/debian/ stretch main contrib non-free
Next, execute the following.
sudo apt-get update
sudo apt-get install libwebkitgtk-1.0-0
wget [http://archive.debian.org/debian/pool/main/libj/libjpeg8/libjpeg8_8b-1_amd64.deb](http://archive.debian.org/debian/pool/main/libj/libjpeg8/libjpeg8_8b-1_amd64.deb)
sudo apt install ./libjpeg8_8b-1_amd64.deb
Now bin/sdkmanager should run and you can install any SDKs you want.