All evaluations result in error or a complex result

I don't see that error anywhere in your worksheet.

However, regarding your definition of i(n), v.k is a range variable, and you can't use a range variable like that. It's not clear to me exactly what you want to do though. If v.k were a vector then i(n) would return multiple values for any given value of n (the number of values being equal to the number of elements in the vector). Is that what you want, or something different?

Yes, I want to plot values of i(n) versus vk(range values from -150e-3 to 150e3).

where n is the state variable which is computed from f(n) :=find(n)

when i try to plot i(n) versus vk(range variable). I am getting error as

All evaluations resulted in either an error or as a complex result.