Multiple Range Variable

Outside of the function definition you have defined Rac as a value with units.

You've defined

H(w,Rac):=....

Here w and Rac are parameters to the function H. There is NO relationship between the parameter Rac and the variable Rac that you defined before. But the variables L and C are defined before the function, with value and unit. The function definition uses those variables, and their units, and finds that the definition of H is not unit-balanced. So you must add a unit to the parameter Rac within the function definition to correct that. That subsequently means that when calling H (as you do in the plot) you have to provide the resistor value for the parameter Rac without unit, otherwise you get another unit-unbalance.

If, instead, you had defined the function H as:

H(w):=....

Then the function would be defined with the variable Rac, as defined before, outside of the function, but that makes it more cumbersome to plot the function for various values of Rac.

The most versatile method is to define H with all parameters, like:

H(w,R,L,C):=...

Then you can define the values for w, R, L, and C and plot it.

Success!
Luc