common was missing db admin user pass in default vars.

roles/common/defaults/main.yml

 secret_name: 'secret'
 secret: '{{ secret_root + "/" + secret_name }}'
 
+db_admin_username: 'postgres'
+db_admin_password: "{{ lookup('password', secret + '/' + 'db_admin_password', length=32) }}"
+
 # let's encrypt
 letsencrypt_server: "https://acme-v01.api.letsencrypt.org/directory"
 

roles/mailserver/defaults/main.yml

 secret_name: 'secret'
 secret: '{{ secret_root + "/" + secret_name }}'
 
+# must match values in roles/common
 db_admin_username: 'postgres'
 db_admin_password: "{{ lookup('password', secret + '/' + 'db_admin_password', length=32) }}"
 

roles/news/defaults/main.yml

 # this is the sha512 hash of the desired password
 selfoss_password_hash: "{{ lookup('password', secret + '/' + 'selfoss_password_hash', length=32, crypt='sha512')|hash('sha512') }}"
 
-# must match values in roles/mailserver
+# must match values in roles/common
 db_admin_username: 'postgres'
 db_admin_password: "{{ lookup('password', secret + '/' + 'db_admin_password', length=32) }}"