installing the HP Prime on Android

[Arno K]

Development options must be enabled on your Android device, not the prime app.
Settings About the Phone, Tap several times on the build number when it was often enough those ( Huawei, Samsung: 7 times, I think) options are available from the Settings menu.
But I doubt that this will help, perhaps you should delete some unwanted stuff from your phone, 400 MB seems very low.
Arno