You can't cancel your own registration
There is no cancel button, on purpose. What to do instead, and why it matters that you do it.
You signed up for something and now cannot go, and there is no button to undo it.
That is not a fault and you have not missed it. Offision has no self-cancel for events, for anybody — the app tells you as much: To withdraw from this event, contact the organiser.
What to do
Message whoever is running the event and ask to be taken off. They can cancel your place in a moment.
The organiser’s name and contact details are on the event itself, under its contact information, where they have filled it in.
What happens after they cancel you
The event leaves My events and your ticket goes with it. You will not be reminded about it again — reminders skip anybody who has been cancelled.
If you change your mind after that, you sign up again in the normal way, as long as registration is still open and there is room.