.Net Nuget Package install rerturns ssl error

I am trying to use Newtonsoft.Json.Serialization namespace in my C# project. While I was searching on Google, I found out "dotnet package Newtonsoft.Json" is a way to install the package on ubuntu linux, but it returned below error:

writing /tmp/tmpC1RGW5.tmp
error:   The SSL connection could not be established, see inner exception.
error:   The remote certificate is invalid according to the validation procedure. 

.Net version is 3.1. Please help this C# newbie. Thanks.