Why can't I charge HyperJuice USB-C battery pack with another USB-C device (like a MacBook)?

You cannot charge the HyperJuice USB-C battery pack with another USB-C device like Macbook, iMac, Laptop, iPad Pro 2018, USB-C Android, etc... 

Only a USB-C wall charger can charge the battery. 

The USB-C is designed to charge other devices and will only detect wall chargers as a Power Charger.