If what @JoeM suggested doesn't get you there - are you typing the column references?
I see that "Sales" in the data source <> "SALES" in the formula - which isn't necessarily wrong, but both the message and the red highlight indicates that Incorta doesn't know about the column references.
As a general troubleshoot for formula columns I'd add one component at a time. Drag "Units" over and test then type "-" and drag the column for COGS -- test, etc.
Also - drop the results into Excel and check the math. The parens can sometimes make the order of operations non-intuitive so you'll want to make sure Incorta is doing the same calculation you think it's doing.