There are two possible scenarios where a mandate could not be terminated:
The mandate has recently been created (within 5 days)
Although the mandate can be seen in the customer account, the enrolment report has not been received - the mandate is not active, therefore cannot be terminated.
There are rules set against the mandate
The “Date Period” rule sets the period from the effective date in which the mandate cannot be terminated.
To change the rules set for a specific mandate, follow the steps here.