Only active cards can be linked to Apple Pay or Google Pay. To check your card’s status, go to Venn "Cards" and make sure it shows as “Active” before trying again.
If your card is active but linking still fails, try adding another card (even one not issued by Venn). If the issue happens again, it may be related to your device’s security rating, in that case, contact your device provider (Apple or Google) for help improving the device’s risk score.