To use Power banks and DC-DC step up Converter to make Electric Bicycle battery Pack
Your batteries provide a maximum of 216 watts, and likely significantly less in the Real World.
Simply connecting batteries in parallel is a tricky thing to do without damage or catastrophe.  One method of providing a safe method involves using series diodes and/or resistors to "balance" the current and prevent the batteries from charging/discharging each other. Either series diodes or series resistors will further significantly reduce the power from your batter packs.
A DC-DC converter will further severely limit the amount of power you have available. You probably won't find any DC-DC converters in any serious electric vehicle design because they are just not efficient enough. Match the battery voltage to the motor by selecting the right battery and/or the right motor.
Grounding is the least of your concerns. Conventionally, you would select the common side of the circuit (typically the negative bus) and call that "ground" and connect it to the chassis.  But it doesn't have any great significance connected or floating.


The three main show stoppers in your design are:


Your proposed batteries don't provide even HALF the amount of power that your motor wants.  You need a much better match between battery and motor.
Simply connecting batteries in parallel is risky and dangerous. NOT RECOMMENDED without taking proper precautions.
Using DC-DC converter is not efficient enough for anything but a small toy.