Are you sure you're using the same API keys on your server and client?
Your server should be using your (live/test) secret key, and your iOS app 
should be using your (live/ test) publishable key as mentioned here on Stripe
Testing.

https://stripe.com/docs/testing

This Error can also come if you are not using the testing card specified by the
stripe try using 

4242424242424242    Visa    Any 3 digits    Any future date