This is because there are two editions of the software:
Community (Open Source)
Enterprise (supported by Bonitasoft)
The REST API Extension feature is provided on both editions but the development plugin in the Studio is specific to the Enterprise edition (along with many other things).
There are two options:
Continue with the Community edition: you can develop REST API Extensions from your favorite IDE
Bonitasoft empowers development teams with Bonita, the open-source and extensible platform to solve the most demanding process automation use cases. The Bonita platform accelerates delivery of complex applications with clear separation between capabilities for visual programming and for coding. Bonita integrates with existing solutions, orchestrates heterogeneous systems, and provides deep visibility into processes across the organization.