In my case it was the distinction between –
(En dash) and -
(Hyphen) as in:
Add-Type –Path "C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
and:
Add-Type -Path "C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
Dashes, Hyphens, and Minus signs oh my!