The scripture says:
Therefore my people go into exile for lack of knowledge; their honored men go hungry, and their multitude is parched with thirst. (Isaiah 5:13 ESV)
What knowledge did they lack?
The following verse helps here:-
Isaiah 5:24 NWT "Because they rejected the law of Jehovah of armies And disrespected the word of the Holy One of Israel."
They had the Law of Jehovah via Moses but did not follow it, thus is could be said they knew OF God, but did not know him. So its lack obeying God is showing a lack on understanding of "the knowledge" of God, as the Isaiah shows:-
Isaiah 1:3 NWT But Israel does not know me, My own people do not behave with understanding.”