Reference : https://stackoverflow.com/questions/47651525/issue-with-restsharp-installation-in-visual-studio-2013
If anyone encounters this kind of problem in the future. I solved it by installing it via Package Manager Console found in (Tools -> NuGet Package Manager -> Package Manager Console) and running the following command:
Install-Package RestSharp -Version 103.1.0
Notice that the installation for RestSharp version 106.1.0 always kept failing, so I changed the version to 103.1.0. Hope this helps anyone with a similar issue.
EDIT
Thanks to phuzi in the comments, RestSharp version 106.1.0 requires .Net framework 4.5.2, as for version 103.1.0 no dependencies are listed.
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