I've got the same problem on a recent OTA update on my Nexus 9. Unfortunately thes "Nexus Root Toolkit" app didn't help because my Nexus 9 was all stock, no root and the OEM unlock was disabled. The problem got me into a dead end. But I found a solution that helped me out and now my Nexus 9 is alive again. It's simple, but you need to follow the steps thoroughly.
Be advised that your data will be completely gone, but at least your tablet will live again.
Ensure your tablet is fully charged and that you have 'adb' available (here's the official process, or a minimal installation)
Download the OTA image for your device, mine was Nexus 9 Wifi so i picked the Nexus 9 "volantis"
Start your Nexus and you'll see an image of an Android robot with a red exclamation mark (you may also see "No command").
Press & hold the Power button. While holding Power, press the Volume up button once. Then release the Power button.
You will then get into the Recovery Mode screen
I executed a Wipe data/Factory reset, not sure if it's mandatory for this procedure, but I did, it's up to you.
With the device connected with your computer choose the option: Apply update from ADB
Execute the following command in the directory
android_sdk_path/platform-tools:adb sideload path_to_the_ota_image_you_downloaded_on_step_2.zipWait until it gets completed, restart the device and that is it!
Good luck!
It worked well for me on the first shot, I hope it helps you too.
