This usually is a result of having an older version of the Plex Server app installed on your SHIELD. To resolve this:
1) On a PC or Mac, open up your web browser and go to:
2) Log in with your Google account.
3) Select "Install"
4) Choose your SHIELD from the drop down list of devices even if your device is greyed out.
5) Wait a couple of minutes for the update to be pushed to your SHIELD. If your SHIELD is powered off, it may take 5 minutes after you power on your SHIELD for the update to appear on your SHIELD. Once the update has been installed, the issue should be resolved.