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 @ 01:29
Script: /wbblite/thread.php?goto=lastpost&threadid=721&sid=905c0c551225d74652a899b7b4f107cf
Referer: