According to the provided information, it seems that it failed to load "https://nuget.telerik.com/v3/index.json". Please provide information in the ticket in English, as English is the official language for communication here.
I checked our status board for the Telerik NuGet, and there are no ups and downs registered recently. I would recomend you to refer to the following article: Install using NuGet Packages. You may need to Generate an API Key as the Telerik NuGet server requires authentication.
Regards,
Nadya | Tech Support Engineer
Progress Telerik