Keep getting error trying to sign up to Discovery+

[AVR1]

I'm trying to sign up to the 12 month free Discovery+ offer, but when I select the Discovery+ tile in "Get more from Sky", I get this:

I tried calling Sky CS about this, but after asking me some questions they put me on hold for about 10 minutes so I hung up.

Any ideas?

[AVR1]

Just to clarify, I've tried rebooting the box and my network connection is working.