Azure Pipelines NuGet "NU1101: Unable to find package"

NuGet searched every configured source and found no package by that name and version. Either the package lives on a private feed not in the config, the name is wrong, or the authenticated source was missing so only public NuGet was searched.

What this error means

A restore step fails with error NU1101: Unable to find package X. No packages exist with this id in source(s): ..., listing the sources it checked.

Azure DevOps

error NU1101: Unable to find package Contoso.Internal.Lib.
No packages exist with this id in source(s): nuget.org

Common causes

The package is on a private feed not in config

The listed sources show only public NuGet; the internal feed that hosts the package is missing from nuget.config.

Wrong package id or version

A typo in the package id, or a version that does not exist on any configured source, leaves nothing to restore.

How to fix it

Add and authenticate the private feed

List the Azure Artifacts feed in nuget.config and authenticate it so restore searches it too.

nuget.config

<configuration>
  <packageSources>
    <add key="Platform" value="https://pkgs.dev.azure.com/org/_packaging/feed/nuget/v3/index.json" />
  </packageSources>
</configuration>

Correct the package id and version

Read the sources list NuGet printed to confirm the feed was searched.
Verify the exact package id and a version published on that feed.
Fix the reference and re-run restore.

How to prevent it

Keep every required feed in nuget.config.
Authenticate private feeds so they are actually searched.
Verify package ids and versions against the hosting feed.