Unable to cancel a PayPal_Standard recurring transaction
As far as i know, Paypal Standard doesn't support cancellation of recurring payment from within CiviCRM. You need to cancel it from your Paypal account.

If you want to have this ability, you need to either upgrade to Paypal Pro or find a payment processor that support it. 

Although I can't find a documentation that clearly state if the payment processor support the cancellation or not, the ones that are listed as SSL (Users submit their credit card directly on your site) in https://docs.civicrm.org/sysadmin/en/latest/setup/payment-processors/ should be able to do it.