Hello @HELEN TRAN ,
Welcome to Microsoft Q&A forum.
Please open Visual Studio > Tools > NuGet Package Manager > Package Manager Settings > Package Sources > check if any invalid package sources are still list there, if so try to remove them or uncheck them, and make sure that the valid package source like nuget.org
is checked, then retry installing NuGet package.
Besides, you may need to try to clean NuGet cache, see this document for details: Clearing local folders.
Best Regards,
Tianyu
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