You'll be able to manage this plan shortly

Neither of the people reporting this issue have Adobe Stock subscriptions associated with the Adobe IDs they are using on the forums. Locking this thread.

I had no problem to take my trial plan and to cancel it. Your situation is not the standard situation. I even would guess, that when you change the browser or the device, the situation could be unblocked. If it doesn't work, and the state of your account stays as it is, you should contact Adobe customer care. Adobe customer care can be contacted by beginning a secure chat session at https://helpx.adobe.com/contact.html?rghtup=autoOpen. Pop-up blockers need to be disabled, you need to accept cookies! If the chat window fails to open, or is non-responsive, use a different device and/or browser to start the interaction.
Support phone numbers may be found here: https://helpx.adobe.com/contact/phone.html. Kindly note that phone charges may apply.

For support via Twitter:
https://twitter.com/AdobeCare

(see also here: https://community.adobe.com/t5/creative-cloud-services/how-to-contact-adobe-support/td-p/11875703 or here
https://community.adobe.com/t5/account-payment-plan/how-to-contact-adobe-support/td-p/11843852)



Important: Adobe does NOT contact you unsolicited by e-mail or direct message. If you get contacted by direct message from a person, claiming to be an Adobe employee, look at that profile and look, if they bear the “Adobe Employee” marking under their name. Adobe support does not use Skype to give you support.


If in doubt, ask the forum.

Same thing is happening to me. There is no option to cancel my trial. 

Sounds like a temporary site access problem. Have you tried again? Here are the instructions for cancelling: 

https://helpx.adobe.com/in/stock/kb/cancel-membership-subscription-stock.html

You can also contact Adobe directly if the problem persists.