Simply detach the card reader from your device and re-attach it. You should then receive a message that your card reader is detected. Also, if your device is being protected by a case, it's possible that is the reason why the error message may appear, so please remove the case before transacting.
Please note: if you are using a device that is operating on iOS 7.0 or higher, please allow Spark Pay to access the microphone in order to ensure a successful swipe. Make your microphone available by completing the following steps on your Apple device:
1. Tap on the "Settings" icon
2. Tap on "Privacy"
3. Slide the "Microphone" option to "ON" to allow the card reader to be detected
4. Close and then relaunch the Spark Pay app