What card reader do you have? If you have a mag stripe reader, does the phone recognize that something has been plugged in? If you have a chip reader, does it show at the top of the screen? The phone can recognize that you've plugged in the reader, but it will only show up if it's charged and running correctly.
https://squareup.com/help/us/en/article/5203-square-magstripe-reader-troubleshooting
https://squareup.com/help/us/en/article/5412-square-chip-card-reader-tips-and-troubleshooting
