I am unable to update any existing Episerver CMS packages or install new packages like EPiServer.Marketing.Testing through Packages manager console. I am currently using visual studio 2017 and tried restarting Visual studio and also cleared nuget cache. Please have a look at below details of error. Package source is set to 'All' and default project to current project.

PM> Install-Package EPiServer.Marketing.Testing -ProjectName EPIAlloyDemo
Install-Package : Unable to find package 'EPiServer.Marketing.Testing'
At line:1 char:1
+ Install-Package EPiServer.Marketing.Testing -ProjectName EPIAlloyDemo
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [Install-Package], Exception
+ FullyQualifiedErrorId : NuGetCmdletUnhandledException,NuGet.PackageManagement.PowerShellCmdlets.InstallPackageCommand

When trying to update CMS to highest minor:

Update-Package EPiServer.CMS -ProjectName EPIAlloyDemo -ToHighestMinor

Attempting to gather dependency information for multiple packages with respect to project 'EPIAlloyDemo', targeting '.NETFramework,Version=v4.6.1'
Update-Package : Package 'EPiServer.CMS' is not found in the following primary source(s): 'https://api.nuget.org/v3/index.json,C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\'. Please verify all your online package
sources are available (OR) package id, version are specified correctly.
At line:1 char:1
+ Update-Package EPiServer.CMS -ProjectName EPIAlloyDemo -ToHighestMino ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [Update-Package], Exception
+ FullyQualifiedErrorId : NuGetCmdletUnhandledException,NuGet.PackageManagement.PowerShellCmdlets.UpdatePackageCommand.

Please let me know if i need to make any changes before running these commands or if i am doing anything incorrectly.

Thank you.