This updated answer was posted Tuesday November 24, 2020 - It's fresh. The link to the bliz Q&A site is at the end there.
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To obtain the Heart of Azeroth you must complete the quest A Dying World. This quest requires your character to be level 50. The quest is pushed to you when you visit your capital city Boralus or Dazar'alor. There is no other requirement.
If you abandoned the quest line in error, go back to Boralus or Dazar'alor and talk to the Earthen Guardian to help you get back to the Chamber of the Heart.
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These fools have a rule against posting links so you will have to figure it out with your brain if you wanna go there
eu.battle.net/support/en/article/176574