Can’t Register AppleCare plus for my iPad

Hi,

I am trying to purchase apple care plus for my iPad Pro which I bought recently.

I have done the following:

• Checked the coverage: https://checkcoverage.apple.com/.
• Entered the page to purchase apple care plus for the iPad.
• Followed the on-screen instructions and finished the diagnosis on iPad. (Followed by accepting T and C, entering billing address and entering the payment details).

However, after I click 'BUY" the page says:

We’re Sorry

This link has expired. Please check your coverage to begin purchasing AppleCare products.

Can someone please help with this issue?