Use an elevated PowerShell (Run as administrator) to install it
https://www.ajtek.ca/wsus/how-to-setup-manage-and-maintain-wsus-part-1-choosing-your-server-os/
Install-WindowsFeature -Name UpdateServices, UpdateServices-UI
If you need to specify the source copy it to somewhere like C:\Temp\WinSxS and use
Install-WindowsFeature -Name UpdateServices, UpdateServices-UI -Source "C:\Temp\WinSxS"