The error is because your vectors x and n are not the same length. x has 501 elements while n has 50. Also, power is doing n.^x. From how you worded things, perhaps you want x.^n?
Assuming you want every n to be raised to every x, I would look into using meshgrid. You can set it up so the rows represent unique values of x and the columns unique values of n (or vice versa). Then just sum by each x so you can create your plot.
x = 0:0.01:5; %% range of x values for graph
y = zeta_func(x);
plot(x,y),xlabel('x'), ylabel('zeta(x)'), title('Zeta(x) Graph'),
%% function definition
function zeta_val = zeta_func(x)
%% zeta(x) = summation of 1/x^n
%% here summing from n = 1 to 50
n = 1:1:50;
% create meshgrid, with unique x in each column, unique n in each row (50x501)
[X,N]=meshgrid(x,n);
terms = 1./power(N,X); %% performing 1/x^i for all n
zeta_val = sum(terms); %% summing the values
end
