If that really is the formula in your cell, I wouldn't expect it to show
#DIV/0! It should flag an error as you have mismatched parentheses.
You may have intended =SUM(B38:O38)/(COUNT(B38:O38)-COUNTIF(B38:O38,0)) ?
In future, don't try to retype a formula from your worksheet to the
newsgroup and risk typing errors. Just copy from the formula bar and paste
here.
If my guess as to what formula you were trying to use was correct, you could
avoid the #DIV/0! by using
=IF((COUNT(B38:O38)-COUNTIF(B38:O38,0))=0,"",SUM(B38:O38)/(COUNT(B38:O38)-COUNTIF(B38:O38,0)))
if you want to show a blank if there are no non-zero values in your range.