Follow this blog to install those module/package first, then try to install 2.0.6-Preview5 later:
[Net.ServicePointManager]::SecurityProtocol = [Net.ServicePointManager]::SecurityProtocol -bor [Net.SecurityProtocolType]::Tls12
Install-PackageProvider -Name NuGet -Force
Install-Module -Name PowerShellGet -Force -AllowClobber
Please note: you need to close all PowerShell Windows and from an elevated PowerShell session to install them one by one. (run this first command, then close current PowerShell window, then open a new PowerShell session to run this second one)
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