Under new SCA (Strong Customer uthentication) guidelines all online transactions are now mandated to use 3DS2 (with sime exceptions for smaller amounts). 3DS2 has been introduced to improve the online payment experience while still protecting the merchant from card fraud. As a result, you will notice that you will now always need a unique code provided by their card issuer to successfully complete a transaction.
Different card issuers will have different ways of delivering the code to you for them to authenticate the transaction. Normally, the bank will send you a code via SMS which they can easily input, or you will be presented with a seperate screen that you will have to passs through to complete the payment process.
However, if you have not received a code or anyother way to authenticate, this could be due to your bank having outdated details such as an incorrect phone number, or perhaps there is a problem with the text not delivering to the user's phone. You should contact your card issuer for further guidance & update details where necessary.
(This only applies for EEA-based merchnats with EEA-based customers. So if the user paying the request is outside of this region this will not affect the payment flow.)