I saw that insync for Debian bullseye Version 3.7.6.50362-bullseye installs fine, but makes a mistake in the definition for the repositories. Every sudo apt update gives an error.
The installation of insync makes in Linux mint Debian edition a repository definition for Mint, Elsie, while it should make repositories for Debian, bullseye. Since that is the name of the repositories of insynchq for this version of insync.
You can easily correct that by hand though, by changing the word “mint” in “debian” and “elsie” in “bullseye”.
If the repository is right, it should work. At least it does on my system running Linux mint Debian edition 5.
Hello all!
Thanks very much for commenting on the forums post — we are confirming a new version for deployment and I will be posting the link here as soon as it’s available 
Hi all,
For Debian 11 users, please give this build a try: New Insync version: 3.7.9
Let us know how it goes!