Hi Tomoaki,
Welcome to the Arkio forum, and thanks for letting us know about this issue with exporting files from Revit via Arkio back to Unity.
The Unity export workflow is primarily intended for Unity users who push their Unity scene with changes back from Arkio to Unity. In this case, the Revit file loaded in the Arkio scene is not supported and removing it from Arkio before you export your scene to Unity should fix this issue.
We will look into adding support for loading imports from our other supported design tools to Unity, so you can expect an update for that on the Unity plugin in the coming weeks.
Let us know if you run into anything else with Arkio 