Reproduce Steps:

1. Clone Unreal.js:
   B:\Documents\Unreal Projects>git clone https://github.com/ncsoft/Unreal.js
   Cloning into 'Unreal.js'...
   remote: Enumerating objects: 6, done.
   remote: Counting objects: 100% (6/6), done.
   remote: Compressing objects: 100% (6/6), done.
   remote: Total 4855 (delta 2), reused 0 (delta 0), pack-reused 4849R
   Receiving objects: 100% (4855/4855), 37.00 MiB | 7.15 MiB/s, done.
   Resolving deltas: 100% (2021/2021), done.
   Encountered 51 file(s) that should have been pointers, but weren't:
   Examples/Content/Color.uasset
   Examples/Content/Demo.umap
   Examples/Content/ExampleBlueprint.uasset
   Examples/Content/Geometry/Meshes/1M_Cube.uasset
   Examples/Content/Geometry/Meshes/1M_Cube_Chamfer.uasset
   Examples/Content/Geometry/Meshes/CubeMaterial.uasset
   Examples/Content/Geometry/Meshes/TemplateFloor.uasset
   Examples/Content/Hello2048.umap
   Examples/Content/HelloBlueprint.umap
   Examples/Content/HelloCanvas.umap
   Examples/Content/HelloCharacter.umap
   Examples/Content/HelloGame.umap
   Examples/Content/HelloInputBinding.umap
   Examples/Content/HelloProcess.umap
   Examples/Content/HelloReplication.umap
   Examples/Content/HelloRest.umap
   Examples/Content/HelloScope.umap
   Examples/Content/HelloSnippet.umap
   Examples/Content/HelloSpringy.umap
   Examples/Content/HelloTimer.umap
   Examples/Content/HelloUMG.umap
   Examples/Content/HelloWorld.umap
   Examples/Content/Mannequin/Animations/ThirdPersonIdle.uasset
   Examples/Content/Mannequin/Animations/ThirdPersonJump_End.uasset
   Examples/Content/Mannequin/Animations/ThirdPersonJump_Loop.uasset
   Examples/Content/Mannequin/Animations/ThirdPersonJump_Start.uasset
   Examples/Content/Mannequin/Animations/ThirdPersonRun.uasset
   Examples/Content/Mannequin/Animations/ThirdPersonWalk.uasset
   Examples/Content/Mannequin/Animations/ThirdPerson_AnimBP.uasset
   Examples/Content/Mannequin/Animations/ThirdPerson_IdleRun_2D.uasset
   Examples/Content/Mannequin/Animations/ThirdPerson_Jump.uasset
   Examples/Content/Mannequin/Character/Materials/M_UE4Man_Body.uasset
   Examples/Content/Mannequin/Character/Materials/M_UE4Man_ChestLogo.uasset
   Examples/Content/Mannequin/Character/Materials/MaterialLayers/ML_GlossyBlack_Latex_UE4.uasset
   Examples/Content/Mannequin/Character/Materials/MaterialLayers/ML_Plastic_Shiny_Beige.uasset
   Examples/Content/Mannequin/Character/Materials/MaterialLayers/ML_Plastic_Shiny_Beige_LOGO.uasset
   Examples/Content/Mannequin/Character/Materials/MaterialLayers/ML_SoftMetal_UE4.uasset
   Examples/Content/Mannequin/Character/Materials/MaterialLayers/T_ML_Aluminum01.uasset
   Examples/Content/Mannequin/Character/Materials/MaterialLayers/T_ML_Aluminum01_N.uasset
   Examples/Content/Mannequin/Character/Materials/MaterialLayers/T_ML_Rubber_Blue_01_D.uasset
   Examples/Content/Mannequin/Character/Materials/MaterialLayers/T_ML_Rubber_Blue_01_N.uasset
   Examples/Content/Mannequin/Character/Mesh/SK_Mannequin.uasset
   Examples/Content/Mannequin/Character/Mesh/SK_Mannequin_PhysicsAsset.uasset
   Examples/Content/Mannequin/Character/Mesh/UE4_Mannequin_Skeleton.uasset
   Examples/Content/Mannequin/Character/Textures/UE4Man_Logo_N.uasset
   Examples/Content/Mannequin/Character/Textures/UE4_LOGO_CARD.uasset
   Examples/Content/Mannequin/Character/Textures/UE4_Mannequin_MAT_MASKA.uasset
   Examples/Content/Mannequin/Character/Textures/UE4_Mannequin__normals.uasset
   Examples/Content/ProxyGameMode.uasset
   Examples/Content/ThirdPerson/Meshes/CubeMaterial.uasset
   Examples/Content/helloJS.umap
2. cd Unreal.js\Examples
3. Open JavascriptPlayground.uproject with UE4.21.2.
4. Alert "JavascriptPlaygound module is missing" will pop up. Select rebuild it.

Result:
Error "JavacriptPlayground could not be compiled. Try rebuilding from source manually." will pop up.

System Environments:
Windows 10 64bit
UE4.21.2 from Epic Games Launcher
Unreal.js 0.3.3 from Unreal Engine Marketplace