But when I try to reinstall the Server after backing up and uninstalling, when I reach the configuration screen. The user authentication method radio box is still disable and local authentication is selected. I can't seems to select to use Active Directory. What could be the problem?
If you missed step 6 in the article you linked, that will cause the symptom you're experiencing. Your old configuration and data are being picked up by the new install attempt, and re-used. You'll need to delete the entire old folder that Tableau Server stored that information in for the switch to work.