You’re not the first to hit this but the docs clearly state that buster is outdated and a prerequisite to running openHABian is to be on bullseye.
Why when there’s 1.8.
On a sidenote, you have an issue with the upgrade process but that isn’t what the thread you posted to is about so I moved it.
No it wasn’t 20. 02? 03?
Check out openHAB 4 migration FAQ