Because Kali Linux comes pre-packaged with hundreds of security tools, Maltego is often installed by default. This leads to the misconception that the software is entirely open-source and free to use without any registration. While Kali is free, Maltego operates on a commercial model, which necessitates a license key—even for the free version.
Maltego offers a free trial, which might be sufficient for your needs. You can sign up on the Maltego website and obtain a trial license key. Maltego License Key For Kali Linux