1. Uninstall the version of AVA you already have on steam.
2. Go to the steam store page and find the AVA page.
3. Reinstall AVA.
4. Run AVA.
5.-This is the part where most people have a problem- Close the launcher and right-click AVA in your game library, and click Properties.
6. Click the Local Files tab and click VERIFY INTEGRITY OF GAME CACHE...
7. You will see a download bar at the bottom of steam, wait until downloading is done. (While downloading you will not be able to use other steam functions.)
8. When downloading is done a window will pop-up that says 23 files could not be verified, but will be retrieved. Start AVA.
9. A few files will download and click through the launchers.
10. GAME START will be clickable.
11. Once you close AVA, you may get an XML error the next time you try to run it again. Go to your steam directory and double click on NWZLauncher to start it again.
If you still cannot launch the game. Copy+Paste your steam "ava" folder contents into the AeiraGames "AVA" folder. Then run from Aeria Ignite.
You can also attempt a complete uninstall and restart which can also correct the issue.
Credits to danteska for figuring out the game cache fix.