The zero error in a spring balance can be found by suspending it freely.
Hold the spring balance in your hand and do not place any weights on it.
Note, where the pointer points.
If it is zero, then there is no zero error.
If the pointer is slightly down (say 1) then zero error is -1. So, whatever weight you measure, subtract 1 from it to get the exact result.
Now, the range of the spring balance is the maximum weight that it can measure. So, if the spring balance has maximum reading as 100, then the range of the spring balance is 0-100 kg.
The least count is found by dividing the range of the scale with the total number of divisions on the scale.
If the total number of divisions on the scale is 100, then the least count is 100/100 = 1 kg.
So, the least weight difference that a spring balance can measure is 1 kg.