one of the my dev org shows "Enable Field Service Lightning" under Setup > Build > Customize > Field Service > Field Service Settings. my another org just have "Enable Work Orders". Is there something I'm missing here?
You will see 'Enable Work Orders' if you don't have the required field services licenses on your org. DE orgs should have the licenses by default. If it's your production org you'll need to contact Salesforce to acquire licenses. If it's a sandbox and your prod org does have licenses you can try to match prod licenses to your sandbox.