We are aware of an issue that affects a small number of users when trying to update the payment card in the app. When entering the last digit of the CVV number, instead of an option to save the screen goes black. This affects a small number of iOS users and can be resolved by following the instructions below.
This issue seems to be caused by the accessibility settings on your device clashing with the app. It looks like having "Reduce" Motion" and "Prefer cross-fade transitions" enabled prevent the card details screen from functioning properly. We'll have this fixed in the next version of the app but in the meantime please check your device to see if you have these enabled. If they are please disable them and try adding the payment card again.
To check
Go to device settings> Accessibily settings> motion> 
If you have these options enabled you'll next see a screen like the one below after tapping 'motion'.
You'll need to disable the Reduce motion and Prefer Cross-Face Transactions.