I've got this error while trying to update Entity Framework tooling from 3.x to 5.0.

dotnet tool update --global dotnet-ef

Gave me the following error

Tool 'dotnet-ef' failed to update due to the following:
The tool package could not be restored.

* You are attempting to install a preview release and did not use the --version option to specify the version.
* A package by this name was found, but it was not a .NET tool.
* The required NuGet feed cannot be accessed, perhaps because of an Internet connection problem.
* You mistyped the name of the tool.

Some of globally defined nuget sources were required an authorization, which actually don't needed for this specific task.

The solution is to remove problem sources from the list, thus first list defined sources:

dotnet nuget list source

And secondly remove the toxic one

dotnet nuget remove source toxicfeed

Helped in my case