Why for Q: 1 6 the answer is 5 6 and not 2 6 ?

Having input "17 11 -1 -4 20 -24"
for 5 6 we have 20 -24 = -4
and for 2 6 we have 11 -1 -4 +20 -24 = 2
why -4 is better than 2?

Because we need to find 2 separate indexes i < j :  |a[i] + a[j]| is minimal.
Not sum of segment |a[i]+a[i+1]+...+a[j]|, but only 2 numbers a[i] and a[j].