Unable to Install Any Package in Visual Studio 2015

I found a solution for this in my case, try to update the NuGet Package Manager.

To do this:

1. From VS, go to Tools -> Extensions and Updates
2. Open the Updates menu option at the left, then select Visual Studio Gallery.
3. If there is an update for Nuget Package Installer, it should show in the list to the right. Click Update
4. Restart Visual Studio

This let me install packages without problem again.

Hope this helps!

Closing and re-opening VS2015 resolves the issue.

It seems that in some cases, simply reloading the affected project will work.

tl;dr - Delete this:

%AppData%/Nuget/Nuget.config

Warning: If you had custom NuGet sources, this will remove them, and you'll have to re-add them.

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Longer version:

You might have corrupted your NuGet config. Oh no :(

Nuget.config is a file used to keep track of all of the places that NuGet pulls from, as well as configuring other things. More likely than not, this xml file got broken somehow.

• Go to this path: C:\Users\{{username}}\AppData\Roaming\
• Delete Nuget.config
• Restart VS for good measure

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For reference: in the good days of 2017, your file should look something like this

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageRestore>
    <add key="enabled" value="True" />
    <add key="automatic" value="True" />
  </packageRestore>
  <activePackageSource>
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
  </activePackageSource>
  <packageSources>
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
    <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
    <!-- Others -->
  </packageSources>
  <packageSourceCredentials>
    <!-- secret stuff -->
  </packageSourceCredentials>
</configuration>