Since I’m so confused, please straighten me up by telling me what is the SI unit of mass. Thank you in advance.
Force or mass? KPa?
Your problem is that your are confusing terms.
The metric unit of weight (or any other force - such as that of a stretched spring, or a bat striking a ball, or a locomotive pulling a train) is called the newton. One newton is defined as the amount of force required to cause one kilogram to accelerate one meter per second every second.