I'm not sure we have enough information to help you here. It would help to see a sample of the data table that you're working with, ideally attached as a JMP data table that we could work with. We really only need the columns in question and only enough data to reproduce the problem.
The error message references "Column 70". Is that the column that contains the formula with the If() conditional? If so, is it a Numeric column or a Character column? Your formula is returning character values, "Red" and "Green", so it should be a Character column.
Also, you don't need the Num() at all in your formula. Because your "(Avg Monthly...)/SAP Corr Forecast" column is numeric the Abs() function should work just fine and returns a numeric value. The Num() function isn't doing anything. I don't think it's hurting anything but it's not necessary.
-Jeff