Powershell load dll got error: Add-Type : Could not load file or assembly 'WebDriver.dll' or one of its dependencies. Operation is not supported

If you downloaded the .DLLs from the internet, Powershell by default will not trust them. You can do one of two things here:

1. Unblock the content. Here's a guide with some elaboration on the problem, but essentially you just need to right-click on the downloaded file, select Properties, and click 'Unblock' in the general tab. You can unblock content directly from powershell by using the Unblock-File cmdlet.
2. Change your execution policy. This can also allow malicious scripts that you download to run, so be cautious.

Specifically reviewing and unblocking content you trust seems the better route, especially for your case since you seem like you would only be doing this once and you trust the package.

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Edit: If that didn't solve that, the other experience I've had with this type of error is when Powershell didn't the same version of the .NET runtime as the assembly. See this question for some context, where Powershell 2.0 ran the .NET 2 runtime and the asker needed .NET 4 for some assemblies.

You're not using the same version of PS so your mileage may vary, but I would try creating/editing a config file per the linked answer to support the runtime of the DLLs you are using.

This worked for me: (from https://stackoverflow.com/a/19957173/107161)

In files:

C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe.config C:\Windows\SysWOW64\WindowsPowerShell\v1.0\powershell.exe.config

<?xml version="1.0" encoding="utf-8" ?> 
<configuration>
   <runtime>
      <loadFromRemoteSources enabled="true"/>
   </runtime>
</configuration>