Hi wang5E73,
it is true that you can't cancel your trial immediately after activation. Please wait a while (up to 3 days) and see if you can then cancel it. Remember that the trial runs for 7 days.
Be also aware that this is mainly a public user-to-user forum. We are not able to help you with any billing, account or subscription issues directly. You will need to contact Adobe Customer Care for such enquiries:
» https://helpx.adobe.com/contact.html
Click the 'circle with a speech bubble in it' icon on the right hand side to initiate a Chat.
Best Regards,
Maurice