You can cancel an upcoming meeting through the admin panel - any upcoming meetings will show on your homepage, and you will see a 'cancel meeting' button at the bottom, next to the 'add to calendar' button.
If you have a one off meeting/mentoring relationship, it will not show a cancel meeting button, but an 'end mentoring relationship' button. This is because the relationship is the meeting, and both are the same, so to cancel the meeting would be to end the relationship also. You can re request a new one off meeting at another time if you do cancel the one off relationship.
Your platform administrator may have disabled the 'one off' meeting option.