Hey, I'm unsure why the following returns an error:
Any ideas on how to fix this?
It means that you already defined K. Entering K = . will remove the previous definition of K, so you can define it again as a function.
Ah thank you!
Not the most helpful error message...
That's because the error occurs after the substitution has been done. If you look carefully,
you will see the presence of [q_, N_] after a bunch of math. That bunch of math is what K was equal to: K[q_, N_] was substituted by <<value of K>>[q_, N_], including some mathematical operations that cannot appear on the left side of := .
It doesn't take long to get acquainted with "Tag [...] is Protected" in Mathematica, and one never forgets what they are due to
I get "Tag" but "Tag Times in ... " is inscrutable. Is "Times" the name of a variable in the OP's script?
No, it's multiplication (in the core Mathematica language).
Separate names with a comma.