Issue with NuGet Packages Not available in this source

Issue with NuGet Packages: "Not available in this source" Message or Error

When I open an existing C# project, I encounter a "Not available in this source" message or error regarding NuGet packages. This issue can be resolved with the following steps:

1. Right-click on the solution in the right-side pane.
2. Select "Manage NuGet Packages for Solution…".
3. In the package source dropdown, where it currently reads "Microsoft Visual Studio Offline Packages", click on the settings icon.
4. Navigate to Nuget Package Manager → Package Source and add a new package source.
5. Label it as "Nuget" and set the source as https://api.nuget.org/v3/index.json.
6. Once again, go to the package source dropdown and switch to "NuGet".

These steps should address the issue and allow you to access the necessary NuGet packages.