Found the problem!!
It is strangely because of a shotcut key conflicts - if there is shotcutkey conflicts between Endnote and anything else it will continue ask you if you wanna save. Symptom: When you click on ‘preference’ in the Endnote Tab in Word and Click OK, it says ‘can’t assign keys’.
Solution: Endnote Tab - Preference - Keyboard - Reset all and OK.