Rule 1: You should attach your worksheet, not only a picture.
You define sigma.tot as a function of x, but technically x does not appear in the function definition, only sigma.nomal and sigma.bending. These two are defined as variables (NOT functions), where the value of sigma.bending depends on a variable called x. But that x is undefined (prior to the definition of sigma.bending). This is how Prime sees your sheet. It cannot and will not vtry to guess what you mean, you have to be explicit in your definitions.