Q. What about this … ?
fastboot command not found
A. Windows is does not include packages for cross platform work with Android. Android developers provides Android Debug Bridge (ADB) for this job and you will need to install the suite platform-tools.
This includes tools that interface with the Android platform, primarily adb and fastboot.
/e/OS documentation provides the page Android Debug Bridge (adb).
The Requirements section of your install page (found from Smartphone Selector) will mention in context the need for the above.