I was having trouble adopting an old Unifi access point. Below is the situation, and what I did to successfully adopt the Unifi AP:
We wanted to add a new Access Point to a client's infrastructure. Their setup had the one and only access point in the middle of the building, and it was not providing ideal coverage to the edges of the office. Most of the work done in said office that requires a strong Wi-Fi connection, is being performed on the edges of the office. After acquiring a new AP, I was to move the current (old) AP to one area of the office, and place the new AP on the other side of the office. This setup will give them optimal coverage where they need it most.
The controller that controlled the old AP was not to be found (old IT group was un-reachable). Because of this, I reset the access point and was going to create a new wireless network, and adopt it to a new controller.
After downloading the latest controller software to their server, the old AP needed to be updated before it could be adopted. So I started that process, but the AP would not update. I spent the next hour and a half forgetting, resetting and attempting to update. Finally, I got the idea that the latest version of the controller software just was not playing nice with this older device.
I had an older version of the controller software on my laptop. I turned off the controller on the Server, and then started the controller on my laptop. Using the older software, I was able to see the old AP, and update the AP, and left it ready for adoption. I turned my laptop controller off, turned on the Server controller, and it was able to adopt the old AP no problem.
In summation, if you are trying to adopt an older Unifi AP, and are using a newer controller version, you may need to load an older controller version to help run updates before a newer controller version can adopt it.
Hope this helps someone in the future!