I have had the following reasons for Emerald Check Out not being available:
1. Additional Driver Added to Reservation
2. Credit Card Expired or About to Expire
3. Driver License Expired or About to Expire
4. Emerald Check Out Closed (Usually middle of the night)
5. Change in Day (Arrived at 11:58 pm and the cannot use Emerald Check out starting at midnight)
6. Confirmation Email Says Counter Service