error NU1101: Unable to find package {name}. No packages exist with this id in source(s): nuget.org

Hi,

I have a .net core 3.1 web api and I want to create an image out of it.
Earlier this web api were referencing a class library project and Nuget restore works and create an Image.
Now referenced class library changed to a private Nuget Package and Nuget restore can not find it.
I select the private nuget source but still getting error
error NU1101: Unable to find package {name}. No packages exist with this id in source(s): nuget.org
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