How to set UI Elements Enabled to true in Applescript?

Question

tell application "System Events"
    if UI elements enabled is true then
        tell application "System Preferences"
            activate
            set current pane to pane id "com.apple.preference.universalaccess"
            display dialog "This script requires access for assistive evices be enabled." & return & return & "To continue, click the OK button and enter an administrative password in the forthcoming security dialog." with icon 1
        end tell
        set UI elements enabled to true

        if UI elements enabled is false then return "user cancelled"
        delay 1
    end if
end tell

When I run this, I get an error. "Can’t set UI elements enabled of application to true"

Any ideas why this may be happening?

Extra info: OS X Yosemite

Answer 1

I just answered this basic question for someone recently. See my answer here.