While the USA admin is doing horrific things at all levels, in this case the guy let his ETSA expire, its a Visa Waiver since he is a British Citizen. ETSA visa waiver is good for 90 days then you reapply online... BEFORE trying to cross the border, or take an international. flight.
The latter part of this article explains more. From personal experience you have to apply way in advance of expected travel date so that if for some reason they don't auto process it, you can apply for a true visa.
https://www.msn.com/en-ca/news/canada/i-want-answers-canadian-firefighter-says-he-was-denied-entry-to-the-u-s/ar-AA1HyF5C