The formula is not working it says I have invalid arguments.

I am trying to establish an error pattern for a comparison. I want to ask Access to look at my database and if there is an error then I need it to calculate an average of four rates so that I can take that figure to a further calculation. Here is my needed end result: The average of the four rate types times the units equals a total that is compared to another total. I am very new and coming from a business area needing to do IT related work that...