The part of your message where you told us what your formula was, and what
values were in the cells to which the formula refers, seems to have got lost
in transit?
The formula is
=IF(ISERROR((B15+B18)-(B16+B18)/(B15+B18)=TRUE),"",(B15+B18)-(B16+B18)/(B15+B18))
Values are B15 = 10
B16 = 1
B18 = 1
Number format is percent with no decimal.