I'm trying to buy a ticket on the app. I have funds in my account and successfully completed the security challenge, but my payment was still declined.

Some higher value transactions (typically over £100) can trigger a further fraud check from your card issuer. This may take the form of an SMS message asking you to reply 'Y' or 'N' to indicate whether it was you making the payment. Transactions with us will be declined until this is completed. Please complete this extra step and retry the purchase. Your card should be approved for use on the First Bus app and you should not see this happen again.