Plot failing for simple equation

See the errors indicated below:

1. You haven't defined a function G. Probably you mean to multiply the constant G with (cos(z(n))-1). Add a multiplication sign.

2 This function has a parameter i, but doesn't use it. Probably you want n instead of i here.

3 The unit dB is undefined. Better leave it out, because you're taking the log already. If you insist on having dB here, define it  (as a unit) with

dB := 1

Then you may get:

Success!

Luc