[System.Web.Security.Membership]::GeneratePassword() - Type Not found
For those landing here who are using PowerShell Core, as per this link:
https://github.com/PowerShell/PowerShell/issues/5352
.NET Core does not support System.Web.dll
As such [System.Web.Security.Membership]::GeneratePassword() will result in the same Type Not Found error in PowerShell Core even with the add-type fix above.
You'll have to roll your own password generator.
Load the assembly first:
add-type -AssemblyName System.Web[System.Web.Security.Membership]::GeneratePassword(10,0)