Sum not working in a cell

To begin with, if you have =C24-C26 (you dont need to use SUM) IN C26, then you are creating a creating a circular error, and that could result in a;; sorts of unexpected answers

If I assume that should be C24-C25, then the arithmetic kind of looks correct -- If I assume that you have a number format that suppresses displaying the difference between positive and negative numbers (something like "0;0"):

C24-C25=5101-6179=-1078 -> if display suppresses the minus sign or other negative indicator, then this might display as 1078.
C26+C27=-1078+58=-1020 -> again, if display of the minus sign is suppressed, then this shows up as 1020.

I don't know why you would have a number format that suppresses display of any indicator of negative values, but, with that assumption the arithmetic looks correct.

I think RJ1969 has some confusion. As per my assume
c24 is = 5101
c25 is = 6179
c26 is a formula =SUM(C25-C24) which is 1078
c27 is = 58
c28 is a formula =SUM(C26:C27) which is 1136