Debian OS:- Linux roblaptop 5.8.0-2-amd64 #1 SMP Debian 5.8.10-1 (2020- 09-19) x86_64 GNU/Linux This is just a development box.
initdb is in the postgresql-13 package, and createdb is in postgresql-client-13. Perhaps you only installed/upgraded one of them?
Since you are in debian, you should also look at using pg_createcluster instead of raw initdb. It'll set things up properly with service starting and permissions and such things automatically for you there.