So U is a column vector
so z is a column vector with the same size as U
requests that the values in z have a trapazoid summation done along the third dimension, using the content of U as the coordinates along the third dimension. However we know that z is a column vector, so its length along the third dimension is 1, which does not match the length of U.
If it happens that x, y, z form a regular cuboid with value U at each point... then you do not have enough U values.
So we have to deduce that you do not have a regular grid: you have scattered points. You cannot use trapz to meaningfully do volume integration in such a case.
Perhaps you could reshape your x, y, z to form a regular cuboid. If so then you would need to know the order they are in, and you would need to know the marginal lengths of the edges. You would then reshape, such as
marglen = [17 11 16];
X = reshape(x, marglen);
Y = reshape(y, marglen);
Z = reshape(z, marglen);
UU = reshape(U, marglen);
and once that was done it would become meaningful to start using trapz over the various dimensions,
trapz( squeeze(X(:,1,1)), trapz(squeeze(Y(1,:,1)), trapz(squeeze(Z(1,1,:)), UU, 3), 2), 1)
Here the intent of the squeeze() section is to extract the marginal unique values for each of the directions.