Once travel has started, the answer becomes more restrictive. Some sections may no longer apply, some cancellation-style benefits may end, and late requests may move from “refund/cancel” territory into claims or curtailment territory instead.
This is why support content should never oversimplify post-start situations. After departure, the issue is often not “Can I cancel?” but “What part of the cover still applies and what evidence is required now?”
If the trip has begun, explain exactly where you are in the timeline and whether you already used any support or treatment.