Thanks to Alexander this is solved.
for PSQL 15 you need extra grants for the user:
GRANT ALL ON DATABASE mattermost TO mmuser;
ALTER DATABASE mattermost OWNER TO mmuser;
GRANT USAGE, CREATE ON SCHEMA PUBLIC TO mmuser;
Thanks to Alexander this is solved.
for PSQL 15 you need extra grants for the user:
GRANT ALL ON DATABASE mattermost TO mmuser;
ALTER DATABASE mattermost OWNER TO mmuser;
GRANT USAGE, CREATE ON SCHEMA PUBLIC TO mmuser;