Add-SPShellAdmin: You need to have Machine administrator priviliges to run this cmdlet

While trying to add a new user to grant PowerShell script execution policies, Got this error message:

Add-SPShellAdmin: You need to have Machine administrator priviliges to run this cmdlet

Fix:
Although you are in  local server Administrator group, You must run the SharePoint Management Shell as Administrator. Simply right click on SharePoint 2010/2013 Management Shell  >> Choose "Run as a administrator"

Alternatively, you can disable the UAC to avoid these kind of issues.

Related article: How to disable UAC in Windows 2012