One does not have a debit/credit card at all (e.g. they're young, or don't have enough documents, or are an immigrant from a sanctioned country).

Alternatively, the card is rejected because "fraud prevention", see e.g. https://news.ycombinator.com/item?id=46424584

Or the game is not available in my "account's region", which is chosen arbitrarily based on God knows what.