This means there is already a monthly or annual subscription connected to your Apple ID. You may want to try the following steps to recover your subscription in order to use the app.
1. From the subscription offers page, tap "$0 Current Subscribers"
2. On the next screen, tap the orange, "Recover your subscription" link found below the two big buttons.
3. You should be prompted for your Apple ID password. This process seeks out the subscription purchase receipt to entitle your iPad.
4. Upon success you should get a "Congratulations!" screen. Tap the "Start Reading" button.