Hi @Paul Leung,
Next-gen only works for the project type "Software".
It was created to be more simple since we spent more time when creating Kanban and Scrum projects, so it will not be possible to convert the project to Business and Service Desk because these templates are not available for these products.
Please, check our documentation about next-gen projects:
- Working with next-gen Software projects
If you have any other question, please let us know.