Did you call your function function? This is a VERY BAD idea, since function is a reserved key word.
Assuming you have simply replaced the name of the function you want to call with 'function' in your example: You need to define input and output in the function definition. For example, for a function called 'myFun', which accepts F-P as inputs, and should return A-E as outputs, you write as the first line of the function
function [A,B,C,D,E] = myFun(F,G,H,I,J,K,L,M,N,O,P)
EDIT
To clarify: You get the error because you're asking for more output arguments than the function can supply. You'd get the same error if you'd call [u,v]=sin(0), since sin is defined with 1 output only. Thus, you have to check your function's signature to solve the problem.
EDIT 2
Let's make an example
I open the editor and define the function
function [A,B,C] = myFun(D,E,F)
%# myFun returs the pairwise sums of the input arguments
A = D+E;
B = E+F;
C = F+D;
Then, I save the function as "myFun.mat" on the Matlab path.
Now I can call myFun like so:
[A,B,C] = myFun(1,2,3);
However, if I call myFun with four output arguments, I'll get an error
[A,B,C,D] = myFun(1,2,3);
In fact, I get exactly the error you got, because I only defined myFun with three output arguments.
Note: You can always call a function with fewer than the number of defined output arguments, but never with more.