Your 2nd post already had
Error: /var/lib/postgresql/9.4/main is not accessible or does not exist
The missing directory /var/log/postgresql/ does not look good as it should be there.
So next please check if that directory (still) exists on your server: ls -lh /var/lib/postgresql/9.4 /etc/postgresql/*
My guess is that some configuration file is already removed by the upgrade so your old PostgreSQL 9.4 is now missing some files. A healthy PostrgreSQL 11 should look like this:
# ls -l /var/lib/postgresql/11/main/
insgesamt 84
drwx------ 5 postgres postgres 4096 Dez 16 09:53 base
drwx------ 2 postgres postgres 4096 Dez 16 09:54 global
drwx------ 2 postgres postgres 4096 Dez 16 09:53 pg_commit_ts
drwx------ 2 postgres postgres 4096 Dez 16 09:53 pg_dynshmem
drwx------ 4 postgres postgres 4096 Dez 16 09:59 pg_logical
drwx------ 4 postgres postgres 4096 Dez 16 09:53 pg_multixact
drwx------ 2 postgres postgres 4096 Dez 16 09:54 pg_notify
drwx------ 2 postgres postgres 4096 Dez 16 09:53 pg_replslot
drwx------ 2 postgres postgres 4096 Dez 16 09:53 pg_serial
drwx------ 2 postgres postgres 4096 Dez 16 09:53 pg_snapshots
drwx------ 2 postgres postgres 4096 Dez 16 09:54 pg_stat
drwx------ 2 postgres postgres 4096 Dez 16 09:53 pg_stat_tmp
drwx------ 2 postgres postgres 4096 Dez 16 09:53 pg_subtrans
drwx------ 2 postgres postgres 4096 Dez 16 09:53 pg_tblspc
drwx------ 2 postgres postgres 4096 Dez 16 09:53 pg_twophase
-rw------- 1 postgres postgres 3 Dez 16 09:53 PG_VERSION
drwx------ 3 postgres postgres 4096 Dez 16 09:53 pg_wal
drwx------ 2 postgres postgres 4096 Dez 16 09:53 pg_xact
-rw------- 1 postgres postgres 88 Dez 16 09:53 postgresql.auto.conf
-rw------- 1 postgres postgres 130 Dez 16 09:54 postmaster.opts
-rw------- 1 postgres postgres 100 Dez 16 09:54 postmaster.pid
same for 9.4 - if it is vastly different some files may already have been removed by your pg_dropcluster … from your 2nd posing.
If this DB is unused you should be able to just drop the database by using pg-dropcluster 9.4 main
