Hi Jun King,
Thank you for your question,
Unfortunately, it is not possible to use the OBO functionality in the Symphony Develop2 (Sandbox) pod environment. The OBO feature will only be possible to test out if you have your own Symphony pod environment - where you will have Admin Portal access to manage the permission for the Service Account & Application Manifest Bundle File.
From the error you provided - it show that 'OBO is not configured' for the BDK configuration - which would make sense as OBO is not available publically to Developers using the Symphony Develop2 (Sandbox) pod environment for the reason I mentioned above.
To enable 'Sandbox' mode with OBO - it would need the Application Manifest Bundle File to be configured to include the correct parameters. However this will only be available to you if you had access to your own Symphony pod environment with Admin Portal access.
In the below link - you will find the documentation explaining on how you would configure the 'Bundle.json' to enable 'Sandbox' mode.
You can read up more on the OBO Authentication using the following documentation:
https://docs.developers.symphony.com/building-extension-applications-on-symphony/app-authentication/obo-authentication
Please let us know if you have any further questions or feedback.
Kind regards
Kaosar