Nextbit robin recognised on fastboot but not adb shell
If fastboot devices shows your device, adb cannot work (and vice versa).

You need to boot to recovery (TWRP). Then you can push the ZIP via adb push and flash it using TWRP's menu (but fastboot devices then will show nothing, which is fully normal).

If ADB doesn't work when booted into TWRP, check TWRP's settings whether ADB is disabled. It might also be that, while booted into recovery, your Robin presents a different ID then when in fastboot, so you'd need to configure that (or, in case of Windows, need another reboot for the corresponding driver to update/activate).