Here are a few quick fixes:

• Enable online purchases: FNB app → Cards → Enable "Online Shopping"
• Check your daily online spend limit in the FNB app
• Ensure your 3D Secure OTP is going to your registered FNB number
• Call FNB on 087 575 9404 or use FNB chat if your card is still declined
• As an alternative, try paying via EFT or use a different card