MATLAB: I am currently trying to run a script to plot two graphs but I keep having this error “Undefined function or method ‘greens’ for input arguments of type ‘char’.”

%%% PARAMETERS %%%

lx = 5.0; % x-dimension of the room (meters)

ly = 6.0; % y-dimension of the room (meters)

lz = 4.0; % z-dimension of the room (meters)

V = lx*ly*lz; % Volume of the room

c = 344; % speed of sound in air

p0 = 1.204; % density of sound in air

pref = 20 *10^(-6);

prms = [20*10^-6:100:100] ;

%%% GF as function of Frequency PARAMETERS %%% fd = [0:10:100]; % frequency range

x1 = 0.1; y1 = 0.1; z1 = 0.1; x0 = 0.0; y0 = 0.0; z0 = 0.0; An =sqrt(2);

f01 = 50; % 1 fixed frequency <<

%%Calculated Parameters%%

w= 2 *pi *f01; %%angular frequency

lambda = c/f01; %%wavelength

k = (2*pi)/lambda; %%Wavenumber

%%% N- Values of x,y,z given k%%%%

nx= (k*lx)/pi;

ny= (k*ly)/pi;

nz= (k*lz)/pi;

%%%% P(x,y,z) for phi(r) Calculation -> close to error sensor %%%

    px1 = (x1*pi*nx)/(lx);
    py1 = (y1*pi*ny)/ (ly);
    pz1 = (z1*pi*nz)/(lz);
    phi_r1 = An * cos(px1) * cos(py1) * cos(pz1);

%%%% P(x,y,z) for phi(r0) Calculation -> sensor %%%

px0 = (x0*pi*nx)/(lx); py0 = (y0*pi*ny)/ (ly); pz0 = (z0*pi*nz)/(lz);

phi_r0 = An * cos(px0) * cos(py0) * cos(pz0);

%%eigen frequency%%% c1 = c/2; k1 = (nx/lx)^2; k2 = (ny/ly)^2; k3 = (nz/lz)^2;

k123 = k1 +k2+k3;

fn = c1*sqrt(k123); wn = 2*pi*fn;

%%%%Greens Function %%%%%

n = -1i * w * p0 *c^2 *phi_r0 * phi_r1; %% Numerator

d = (w^2 – wn^2)*lambda; %% Denominator

g = n/d;

%%Sound power Level&&

SPL = 20*log10(prms/pref);

%%Plot Sound Power Level against greens function &&

 figure(1); 
 plot(fd,SPL');
 title('Greens function in a room');
 xlabel('Frequency (Hz)');
 ylabel('SPL (dB)');
 hold on;

figure (2); plot(g, SPL') title('Greens Function within a Room'); xlabel('Greens Function'); ylabel('SPL (dB)'); hold on;