"ERROR: permission denied for tablespace pg_global" in PostgreSQL agent (4375543)

The database agent is missing the necessary permissions as described in the Deployment Guide and Knowledgebase article 4229776 .
 

Something may have changed in the permissions that the user has. Grant these permissions to the user:

User privileges required for the PostgreSQL agent for versions 10+:

• PG_MONITOR
• PG_READ_ALL_SETTINGS
• PG_READ_ALL_STATS
• PG_STAT_SCAN_TABLES

The follow script can be used to confirm the necessary permissions have been applied. Replace USERNAME with the name of the PostgreSQL account used for monitoring the database.

SELECT
r.rolname,
ARRAY(SELECT b.rolname
FROM pg_catalog.pg_auth_members m
JOIN pg_catalog.pg_roles b ON (m.roleid = b.oid)
WHERE m.member = r.oid) as memberof
FROM pg_catalog.pg_roles r
where r.rolname='USERNAME'
ORDER BY 1;

These commands can grant the necessary permissions

GRANT pg_monitor TO USERNAME;
GRANT pg_read_all_settings TO USERNAME;
GRANT pg_read_all_stats TO USERNAME;
GRANT pg_stat_scan_tables TO USERNAME;