TLDR ;)

Ok, let's get started to fix this issue :)

First, you need to navigate the Visual Studio instance and for that do the following:

1. Open Properties on VS shortcut and then click on "Open File Location"

   

2. Edit as administrator the devenv.exe.config with Notepad++ or other editors you prefer:

   

3. Navigate to the end of a file and check if you don't have <defaultProxy>... section:

   

4. Make sure to add following inside <system.net>:

   <defaultProxy enabled="true" useDefaultCredentials="true">
      <proxy usesystemdefault="true" bypassonlocal="true" />
   </defaultProxy>

The final result should be like this:

A few notes:

1. If you have several Visual Studio instances then make sure you edited devenv.exe.config for each one.
2. Sometimes after VS updates, these files content can reset. So a good hint here is that once you Update Visual Studio instance, then go and check the .exe.config file.

Update: Btw, the same kind of approach work for many other applications, for example, check this post.

I got this error using VSTS as source provider (git). Was logged in to a MSDN account in VS2017 based on the correct email address, i.e. same as the one used for my VSTS login. Upon restoring packages from the VSTS nuget-feed, I would repeatedly be prompted to log in to my MSDN-account, but still get 401 from the source/feed.

Turned out, I apparently had 2 different MSDN-accounts on the same email address - 1 "Personal" account and 1 "Work or school" account.

Solution: simply log in with the correct one (which here turned out to be the Work one)

The same error is showing for VS2019. For my case Restart the Visual Studio and it's working fine. In my case, it's the network-related issue, you have a proper Internet connection.

Another method - a workaround - Just do it manually!

I gave up all solutions provided in Github an SO.
Checking the Manage .NET project and item templates docs I just found out that I can install a package locally.

So if anyone is struggling with this, just download .nupkg the package from the source and use the following command

dotnet new -i c:\some-folder\some-package.nupkg

How to download a package manually

1. Go to Nuget.org
2. Search for your package and click to see the details
3. Locate the Download package link, located at the Info section.
4. Use the command shown above. Cheers! Package installed!

Please go to Nuget.org and check if the Nuget is up.