t = 0:1000;
t is a row vector
n = 0:250; % Number of summations
n is a row vector that is a different size than t.
sum_parts = ((3.*(2.*n+1).*pi.*sin(((2.*n+1).*pi)./2))./(((2.*n+1).*pi)+sin((2.*n+1).*pi))) .* exp(-(((2.*n+1).^2).*(pi.^2).*Dab.*t)/(4.*(L.^2))) .* (sin(((2.*n+1).*pi))./((2.*n+1).*pi)); %Summation
That involves row vector n and row vector t, and uses .* between expressions that are length(n) and expressions that are length(t) . With those being different lengths, those are incompatible.
Cal = ((Cao.*Vp)./Vl).*(1-(2./L).*sum(sum_parts,2)); %Final Function
The sum,2 implies you are expecting sum_parts to be either a row vector or else a 2D array. If it were a row vector then sum,2 would give a scalar. If sum_parts is a 2D array then whether the result is length(n) or length(t) depends on whether the array is t x n or n x t.
You plot f(t) versus t, which tells us that you expect the sum to be a vector of length t, either 1 x length(t) or length(t) x 1. And with you having summed sum_parts over the second dimension, we can predict that you intended sum_parts to be length(t) x length(n) and that you want to sum over the different n values.
To have that happen in the context of your code... make t a column vector instead of a row vector, at least inside your function. Such as if you used
t = t(:);
