My guess is that because you were using the card outside of your home country, the PIN request was triggered for identification verification purposes. (money.stackexchange.com)
Therefore a local tech firm has offered to integrate identification verification software that checks if the person is 21 + upon sign up of the event. (law.stackexchange.com)
These «online identification verification systems» are prone to causing false alerts that you aren't really the one taking the course. (drive-safely.net)