Method not found exception (EventWaitHandle..ctor) using WinSCP .NET
You are most probably using PowerShell 6 or newer (aka PowerShell Core). These versions of PowerShell use .NET (Core), not .NET Framework.
So you have to use the .NET Standard build of WinSCP .NET assembly. Inside the WinSCP-*-Automation.zip package, check the netstandard2.0 folder.
Read about Installing the assembly for PowerShell.