how to avoid fee payment and why was adobe stock added by itself?

Try with Adobe customer care. You have probably taken the 30 days free trial subscription for stock and you have not cancelled in time. You now have an annual subscription. The early termination fee is half of the remaining dues.

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.

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 check 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.

Adobe does not subscribe you to any of their services; you must do it yourself. Does anyone else have access to your logon information? You can contact Adobe Customer Service via the contact us link on your account to attempt to renegotiate the cancellation fee.