Please note that the official communication language for the support service is English. So, I will kindly ask you to translate your question in the forum thread for more accuracy. In the meantime, I've used Google Translate to see if I can understand your question. If I understand correctly, the problem is that the .NET Core Project Converter is no available in your Visual Studio menu.
As noted in the article, since R2 2020 Telerik UI for WinForms suite provides tooling in Visual Studio 2019, via our Visual Studio Extensions, that will convert client projects that use .NET Framework 4.8 (or lower) into .NET Core projects. This tooling is only available in Visual Studio 2019. Having said that, is it possible that you use another version of Visual Studio?