I am trying to cancel my plan, but the website won't let me because of a pending billing error. I do not want to continue the service or update my payment method. Can you please cancel the plan from your end and remove my card?

I can’t really answer for the billing issue (only Adobe employees have access to your account, understandably) but sometimes the webchat fails because there’s an ad-blocker or a pop-up blocker. so you could try to disable them momentarily if that’s your case.

You can have other others such as using another browser (FF is built differently than other browsers so if that’s what you are using, there sometimes glitches) or use social media by tagging ​@ adobecare (without the space between @ and the rest)
They operate on X and Instagram. You can DM them directly instead of creating a post.

(apparently, typing “AGENT” gets you a real person first time, I’ll let you test that!)