Bo Pang from Google writes:
"I think there are two issues here:
1. The binary build version of Unreal 4.20(from Epic Game Launcher) somehow is missing the Tools folder under Engine\Plugins\Runtime\GoogleARCore. So If you are using the binary version of Unreal 4.20, you need to copy the Tools folder from either the 4.19 Google's Unreal Engine repo or the 4.20 branch from Epic's Github repo and run the Setup.bat to get the Tools folder.
2. There is an issue in both 4.19(Google's branch) and 4.20 Epic's branch that if you put the Unreal Engine folder in to a path that contains empty space, the Augmented Image data base will failed to build. The workaround for now is to try to remove the path in your Engine folder and your project folder.
Also note that you will need to change the Augmented Image Database asset to make the cooking process happens again."
1. Download Augmented Images example project https://github.com/google-ar/arcore-unreal-sdk/releases
2. Attempt to launch onto device using 4.20.1 from launcher
Result: LogGoogleARCoreAPI: Error: Failed to build augmented image database.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-62277 in the post.