Can't validate product’s purchase date

my device: iPad Pro (9.7-inch, Wi-Fi + Cellular)

I bought my ipad from Tiki store (e-commercial in Vietnam) in October 20th, 2018. And i have a electric invoice.

But, when i access checkcoverage.apple.com page to check guarantee, i have received a notification "Please validate your product’s purchase date.".
After that, i login my apple id and choose October 20th, 2018 for validate purchase date. Again, apple notice "We’re sorry, but we are unable to complete your request at this time. Please try again later."

how to i can active guarantee for my ipad ?