Amex doesn't return balance for supplementary cards, so any calls to the cards/balance endpoint will receive an access_denied error.
This is a design decision made by the provider. The rest of the calls will work as expected.
Solution
If the user is the main cardholder, they can link both cards (main and supplementary) on the same consent.
TrueLayer is advocating with the provider to re-enable the endpoint for supplementary cards, and we will update this article should there be any developments.