You can only cancel active recurring plans, and you (or the donor) can only do so when looking at the recurring plan view of that record.
First, make sure that you are actually on the record page for the recurring plan you are trying to cancel.
If you're not on the Recurring Plan section, then you may either viewing a donation or in a supporter's view.
If you are on the Donation view, please scroll down to click the Recurring plan ID to be taken to the Recurring Plan's record page.
Donation view example:
If you looking at a supporter who has a recurring plan, please scroll down to the section for "Recurring plans" and click the row for the specific plan if there is more than one.
Supporter view example:
Only when you are on a recurring plan page itself, and only if the plan is active, will you see the Cancel recurring button.
If the Recurring Plan is in the status of Failed our system will no longer process further donations, which is equivalent to being canceled.