can't install citext extension: could not access file "$libdir/citext": No such file or directory
Did you perhaps install multiple instances of Postgres on the same server? Standard Postgres is not built for that, $libdir is compiled into the program. If so, you could fix your problem by uninstalling all other instances and and re-installing your current Postgres version.

Related:


Could not access file “$libdir/plpgsql”: No such file or directory