Step 1: Open the payment form and double‑check that the three‑digit security code matches the one on the back of the card. Even a single digit error triggers error code 101, which blocks the transaction.
Step 2: Ensure the expiration month and year are current. Cards past the listed date produce error code 202, causing the system to halt the purchase attempt.
Step 3: Switch to a different browser or clear the cache. Cached cookies can corrupt the request payload, leading to error code 303 that appears only on the original browser.

Step 4: Verify that the issuing bank has activated 3‑D Secure (or equivalent verification). Without this layer the gateway returns error code 404, which prevents the funds from moving.
Step 5: Contact the bank if none of the above resolves the issue. Provide them with the exact error code; most banks have a lookup table that identifies the root cause within seconds.