A cell can contain either data (number/text/both) OR a formula, it cannot contain both. So, if you are putting this in G139, but then also need to text G139 for something, you cannot do that with a formula, without causing a circular error
(IF G139=>E139) then ((K139=G139)*($O$139))
If this was going into a blank cell then the formula would be...
=IF(G139=>E139,G139*$O$139)