How to make a screenshot of UAC dialogs that remove all functionality from Windows (trying to protect you) so that even the Print Screen button doesn’t work, not to mention the Snipping Tool?
Here’s how to enable making screenshots of the UAC dialog
1. Type gpedit.msc in your search box and run it (to configure Windows’s security policy)
2. Navigate to Computer Configuration -> Windows Settings -> Security Settings -> Local Policies -> Security Options
3. Find User Account Control: Switch to the secure desktop when prompting for elevation and double click it
4. Disable the option and click Apply and OK
See also a similar discussion on Stackoverflow