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: 19.12.2018 @ 01:34
Script: /wbblite/board.php?boardid=21&sid=1efc43be62555eeafcd9a22ff4f5da55
Referer: