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Do I need to click "I'm ready" button on or before 12/31 to prevent my order being cancelled or can I just let it expires so I will be back to the queue automatically?
Nobody knows, but from what I have seen, it will likely cancel on or after 12/31 so I suggest you click it before unless you want to take a gamble with your order being potentially cancelled.
Same for my account. Was planning to click on 12/30 late in the day. My SA said to check back 1/1 or call early next week. Said they’re pushing for Q4 deliveries and that’s why a lot of holds changed on the app but they should refresh after new year.
I’d suggest just sending a message to your SA asking why the option is gone and that you were hoping to take delivery.