Server Requirements To Run Sage Through IIS
Setting up your server to support a Sage integration application
Sage requires the user that the IIS application is running as to have full control of 2 folders: the sage install folder and the licenses folder.
You can find where these are on the machine by launching Sage and finding the Program Directory and Shared Data Directory in About > System Info:
Make sure the application pool for the IIS website has full control for the directories mentioned there.
The IIS account also does not need administrator privileges.
Here is a gif of setting the IIS User for your application pool: