Another user has reported this issue as well - I think it's caused by using an older version of Unity, that uses either an older C# or .NET level that doesn't have support for the WriteAllTextAsync method. Make sure you're using Unity 2021 as recommended in the guides.
I actually played around with this model a bit when testing the importer, but unfortunately it wasn't very well suited for use outside of Blender. The wings in particular use some mesh deformers rather than bones, which can't be exported to FBX in a way that works in Unity. Someone would need to spend a good bit of time re-rigging the wings, and maybe making other rig adjustments, before it could be used in the importer.