It looks like you're using an older card reader, the Magtek iDynamo. We'd strongly encourage you to upgrade that reader to the BBPOS Chipper 2X BT.
It looks like the card reader is being a little finicky. Make sure to hold the card and phone firmly, and swipe the entire card through the card reader. The card reader needs to read the entire black magnetic strip on the back of the card.
It may be helpful to swipe the card slower, to ensure that the full card passes through the reader. Also, be aware of your hand position holding the device, as we know it can be difficult to swipe and miss your wrist at the same time!
Lastly, you can try swiping the opposite direction as well. The card reader will work either way.