The first remark is that names of built in functions must not be used as variables ( trapz in this case ), second remark is int function is for symbolic computation, so you need to use symbolic variable. Your solution is good enough, here is an enhanced version using only 100 as limit :
n=[10:10:100];
syms X
F=int(sin(X),0,pi);
% F equals 2 but F is symbolic again, so you take value
t2=2*ones(size(n));
for i=1:length(n);
x=linspace(0,pi,n(i));
t(i)=trapz(x,sin(x));
error(i)=(t(i)-t2(i));
end
semilogy(error)
