I'm trying to sign-up to Kindle Unlimited via the 30 day trial but when I click through I'm issued with an Error 400 message.
I've managed to purchase something else on the site today so I don't think this is an issue with my account or card.
Welcome! Try using this link to sign up for Kindle Unlimited. If you haven't had a free trial you'll automatically be signed up for that, and if you decide to keep Kindle Unlimited you'll be enrolled in the paid plan after the trial ends.