Same problem in Unreal 4.26. I get a total of three Errors when building.
PackagingResults: Error: Expected AkSpatialAudioHelper.h to be first header included.
PackagingResults: Error: DoesPackageExist: DoesPackageExist FAILED: '' is not a standard unreal filename or a long path name. Reason: Path should be no less than 4 characters long.
PackagingResults: Error: Unknown Cook Failure