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The turnaround time of responses are the main issue here, but you’re doing everything correctly. Quoting CFA Institute:
Withdrawal is not a self-service option right now, but candidates can withdraw by reaching out to our contact center. We disabled the self-service functionality because candidates were misunderstanding what it meant to withdraw and accidentally cancelling their registration. We hope to have the self-service functionality reinstated in the future.
Some candidates have had luck calling up and getting the withdrawal processed on the spot, but otherwise it’s a waiting game.
Unfortunately missing the 20th July deadline might be collateral damage, unless you can get CFA Institute to allow registration past the deadline (which they are allowing candidates awaiting May’21 results to do).