Can you please provide additional information on what specific error message you received? Any further details are appreciated so I can present accurate information.
In the meantime, I recommend accessing your account using a private window and log in from there. There are times when a browser's cache data becomes full of frequently-accessed page resources or damaged. This causes unusual behavior in the account like unable to cancel subscription or transactions. And, private browsing doesn't store local files or cache.
You can refer to this article to see different information about the canceling subscription in QuickBooks: Cancel your QuickBooks Online subscription. This also provides a link on how you can resubscribe your account in the future.