We had this same issue on the reservation we made oodles ago. It wouldn't let me check in online. So, I called and talked with the agent at Alamo. She couldn't get the reservation to open either. So she connected me to Customer Service. After about 5 minutes on hold, she came back and said they couldn't open it either. They then connected me with the escalation department. After about a 15 minute wait I got someone from that department. He said that coupon code is no longer valid and that is why the reservation would not pull up. I then told him well, per the AOPA website,and that I AM a member of AOPA, the coupon is valid through 12/31/18 right on the AOPA/ALAMO website. He said from what they know the coupon is no longer valid. He did say though that the reservation is still good, and they confirmed our rate. He said the only thing we couldn't do is check in online, which means we have to go to the counter when we get to orlando. We cannot simply go out to the garage, get our car and simply drive away.
So we'll see. We have a different reservation with both ALAMO and BUDGET for just a little more than what this would have been with the coupon at Alamo. If the lines are too long at the counter, it might be worth the difference to just pay the difference and maybe go with another company. My time is more valuable than standing for hours at ALAMO to go to through their counter.
~D