As mentioned in the error, that y is non-tunable, so have you tried to select the 'Variable Size' checkbox by specifying the upper bounds in the Size box?
Also, find function will return the index of the non zero elements in a vector, so if you want values at the index, you can code in the following way:
function y = non_zero(u)
x = find(u); % will return index of non-zero elements
y = y(x);
end
