SQL-DATABASE ERROR

Database error in WoltLab Burning Board: Link-ID == false, connect failed
mysql error: Can't connect to MySQL server on 'mysql.service' (113)
mysql error number: 2003
Date: 23.10.2018 @ 15:04
Script: /wbblite/profile.php?userid=42&sid=82e247a6f85d3f64dcb29cd5e727e99e
Referer: