If you are using some keyboard language other than English, that is a possibility. I also found this answer to your question doing a search: The Right ALT has been commandeered by a Graphic Driver control panel. There should be a corresponding item in your notification tray that allows you to enter and reconfigure the keyboard preferences so that your Right ALT is allowed to act normally.
Hiiii..  I am using Vostro 3501. I want to make Right alt key (Alt Gr) to work same as Left Alt key. Is it possible to do..? Please clarify. Thanks.
Is there a way to use Right alt key same as left alt key