Unless you're using Debian, where people care about this, chances are that setting up GPG Smartcards with your Linux distribution is an adventure. There are two ways to do it, CCID and PCSCLite, the latter of which runs a background service pcscd — as root, at least on Arch, while GnuPG's built-in CCID driver accesses the reader directly through the USB device, but took me a lot longer to figure out how to use.

Problem

You're getting a "card error" when trying to access the card. The same thing works as root.

$ gpg --card-status
gpg: selecting openpgp failed: Card error
gpg: OpenPGP card not available: Card error
$

When trying the same as root, you can access the card.

Solution

Add a udev rule that changes file permissions for you, as documented on https://wiki.archlinux.org/index.php/GnuPG#Smartcard_not_detected

Add your user to a new user group that can read smartcards, and give that group read-write access to the device when it's plugged in, by creating a file /etc/udev/rules.d with the rule:

ACTION=="add", SUBSYSTEM=="usb", ENV{ID_VENDOR_ID}=="08e6", ENV{ID_MODEL_ID}=="3438", MODE="660", GROUP="scard"

(Figure out the values for the vendor and model ID using lsusb, they are printed separated with a colon.)