That is due to the limitations of all hardware encoders at the present time. No hardware currently supports 2-pass VBR at all. You'll have to use only software encoding if you want VBR 2-pass.
And no hardware encoder will let you set an extremely low bitrate, for that matter: The image quality from hardware encoders at such low bitrate settings is downright abysmal, far worse than software-only encoding.
And I am assuming that you're running the 14.2 beta version of Premiere Pro. Because all current non-beta versions officially support hardware encoding only from Intel IGPs (or integrated Intel CPUs' HD/UHD/Iris Graphics). In other words, officially only Intel QuickSync is supported for hardware encoding.