If you are trying to connect your Amex account more than once on Money Dashboard Neon (for example, you have multiple Money Dashboard Neon accounts/your partner also has access to your American Express account), this is currently not supported and hence, causing the failure.
If this isn't the case, please get in touch with the support team by submitting a help request here.
Bank accounts, American Express (Amex)
Relevant to users on:
Money Dashboard Neon
Not sure which App you are on?
If you are unsure if you are on Money Dashboard Classic or Money Dashboard Neon? You can find that information here