There may be many reasons why you can't use a card. To sum up, there are roughly the following categories:

1. card status is abnormal, such as card degaussing, too many password input errors, hotline confirmation.

2. If the card is not connected to the network, the transaction will fail.

3. If you hold IC chip credit, if the cashier still operates the transaction by swiping the card instead of inserting it, it is also unusable.

4. Cards do not support transactions, such as V-ID cards, UnionPay transactions or some online transactions.