The System Requirements Test is a 3rd party script, available at UBBWiki. The test will also tell you if they're running PHP7, which is not supported at all at this juncture (though you'd think that the missing mysql_connect command error would be shown if it was PHP7)

The error message you supplied is saying that the forums cannot connect to the database, the error is server related:
Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (11 "Resource temporarily unavailable")

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