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: 20.10.2018 @ 20:16
Script: /wbblite/board.php?boardid=2&sid=47094dbe778c1358aa1b61af57f4d63e
Referer: