*Repo steps for command line:*
1.1 Download the attached project
1.2 Delete all but "Assets" folder in the project (This is a requirement for user for some other reasons)
1.3 Run command line (Unity.exe -projectPath "C:\dev\UnityProjects\Unity56\UnitySdk_TC" -quit -batchmode -executeMethod PlayFab.Internal.PlayFabPackager.MakeAndroidBuild -logFile "C:\dev\UnityProjects\Unity56\buildAndroidOutput.txt" <- make sure that project is in correct folder and cmd is in correct unity folder)
Result: <Build FAILS>

1.1 Open project in the same Unity version
1.2 Set the identifier to empty string
1.3 Save the project (File->save project (important)) Note:it's not actually set to "com.PlayFab.PlayFabTest" either
1.4 Run command line (as described before)
Result: <Build SUCCEEDS>

*Repo steps in Editor:*
2.1 Delete all but "Assets" folder in the project (This is a requirement for user for some other reasons)
2.2 Open project in editor (You can examine the Android identifier settings,just don't change anything or save)
2.3 Build on Android from PlayFab/Testing/AndroidTestBuild
Result: <Build FAILS>

2.1 Set the identifier to empty string
2.2 Save the project (File->save project (important)) Note:it's not actually set to "com.PlayFab.PlayFabTest" either
2.3 Build on Android from PlayFab/Testing/AndroidTestBuild
Result: <Build SUCCEEDS>

Reproducible: .5.6.0b6, 5.6.0b10, 2017.1.0a2
Not reproducible: 5.5.2p2, 5.6.0b5

Build fails because BundleIdentifier is not set before building, but it should be set in PlayFabPackage.cs line 123

By Design: the way application identifier works has been changed in Unity 5.6. Now it has to be set separately for every platform that uses it and you should use PlayerSettings.SetApplicationIdentifier(BuildTargetGroup, string) to do that.
from 5.6 there are basically two options:
- change the platform before setting the id by launching unity with '-buildTarget <name>' batch mode argument.
- use PlayerSettings.SetApplicationIdentifier to set the bundle id