Perhaps you have a formula in B8 or C8 that returns "0" rather than 0.
Note the first of these is a text value and this will give you the
#VALUE error if you try to use it with arithmetic. Or, it could be
that a formula in G8 is returning a number enclosed within quotes, and
again this is a text value and will give that error.
Only you know what formulae you have, as you have not shared those
with us.
Note that you don't really need the SUM in this case - you could write
your formula as:
Empty cells won't cause a #VALUE error with this formula. You should only get
a #VALUE error if any of the cells (B8, C8, or G8) has a formula that
evaluates to #VALUE, or if G8 contains text (a non-numerical value).