The absolute value of a number, is defined as the distance from such number to the zero.
You can notice in the given image that the distnace from -8 to 0 is 8 units. Then, you have:
|-8| = 8
the absolute valuo of any positive or negative number, is a positive number.
The error is that |-8| is not equal to -8. It is equal to 8.
