My absolute favorite bug is the one where the “cancel your Amazon Music subscription” is seemingly the only one of what must be tens of thousands of UI elements in the Amazon app that simply loads a blank white screen no matter what.
What are the odds!
If a website won't let you cancel, call your bank to force them to block the payment. This will cost the website $20 and a ding on their credit report. Be aware the website will ban you, so you'll need to make a new account.
Sounds like credit card sites. Most everything is pretty creaky; “redeem rewards” is broken or offline for maintenance; “pay bill” is greased lightning all the way through.