The .NET scripting backend uses Microsoft’s .NET to power scripting. Unlike other Unity platforms, Universal Windows Platform does not use the .NET 3.5 Class Libraries profile, but rather a subset of .NET 4.5. This is due to Windows Store certification requirements. This means that the platform has some incompatibilities compared to other platforms, such as different plugin model and many missing types that are present in .NET 3.5 profile that Unity normally uses.
ノート .NET スクリプティングバックエンドは非推奨で、今後リリースされる Unity からは廃止される予定です。IL2CPP スクリプティングバックエンド に変更することを推奨します。