We’re unable to validate your product’s purchase date
Solution 1:
What’s the harm in providing the evidence you have that’s best for you?
Apple typically starts the warranty when you complete the purchase and the authorized seller ships the device. That’s when you “bought it” and you control the shipping costs and times in reality, not Apple.
Also, don’t worry about a day or two. If you have issues at the end of the warranty you can arrange service on the last day and then ship it - get another 90 days coverage and even if you present your device a week after expiration, my experience is Apple covers these days / week periods in your favor anyhow.
This form pops up when the date Apple thinks it sold it is too far in the past. If this left the factory recently, this form just starts your coverage today, so already Apple is a little cautious so I wouldn’t put a date here that you don’t have evidence to corroborate in case when you list the receipt date, they ask you to upload a scan of the delivery confirmation.
Unless your local law says warranty starts when you receive the gear, the proper transfer of warranty starts when you pay for the goods, not when you receive them.
Solution 2:
I had a similar problem. Kept writing in the date when I purchased the MacBook Pro 16". Which, to be fair, is what the page is asking for.
But that's not what they want to know
When I changed to the date the package actually arrived, which was 2 months later (thanks, Corona virus) it suddenly worked like a charm.
Which begs two questions: 1. Why the obscure message about "not being able to process right now"? When it clearly should read "You entered an incorrect date, please try again"? 2. Why not say "when did you recieve your product" instead of "purchase your product"? We will never know, because Apple.