“I started a trial and canceled immediately. I did not use the service. Now I cannot cancel because of a past due charge. Please cancel my subscription and waive this charge

Only you can camcel.

See here:

https://helpx.adobe.com/manage-account/using/creative-cloud-subscription-terms.html

and here:

https://helpx.adobe.com/manage-account/using/cancel-subscription.html

You need to have a valid form of payment before you can cancel a subscription and get a refund.

https://helpx.adobe.com/account/individual/billing-and-payments/payment-methods/update-payment-information.html