QB Issue Resolution:

If you have a free trial, you don’t have to do anything and it will die it’s natural death.  However, if you put your credit card details in, I would try a private window.  Gear>billing and subscriptions>cancel

Resolution for Issue 'Cancel Trial giving error' available: Yes (Solved).
Source: Intuit Community forum.

Cancel Trial giving error: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for billing and subscriptions
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Error Recovery for billing and subscriptions [Canada]
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