This could be due to the type of card it is; by default, Curve supports VISA and MasterCard debit and credit cards.
We aren't supporting prepaid cards as standard but we may support certain prepaid cards after running a risk review on each card, on a case-by-case basis. At the moment, we allow Osper, Monzo and N26. Revolut is not currently supported.
If you'd like us to look into the possibility of supporting another card, please get in touch!
Alternatively, this could be because the card is added elsewhere on Curve. It could be that you have already added it but it isn't displaying properly- if this is the case, please sign out and sign back in again using a WIFI connection.
If it is a card from a joint account, you won't be able to add it to Curve if another card is already on Curve somewhere - this is an issue we are working to resolve!