How do I fix this?

It’s a problem with either:

A: Your bank account

B: Apple Pay is disabled on your account

Problem B should be easy to fix but problem A will require you probably calling your bank

"Contact the card issuer"

When my card was declined, I decided they knew more BS with the TikTok shop than I do...

So, I'll never even attempt to buy from there.. just cheap China sh1t anyway.

Don’t buy shit off TikTok🤷🏻‍♂️