You didn't define Lp. The bold equals = you use in Lp=log(p/atm), is an equality operator, not an assignment. You should use :=, which you get by typing just a colon :
See the Lp:= to the right.
Next thing is the atm, I assume that's the unit atmosphere. You defined p without a unit. Dividing it by a unit in the argument to the log function will cause another error, since that argument should be unitless. Define p with the unit atm, or remove the division in the log() function.