Maybe it is a syntax problem, since I haven't been using Mathematica for much time, but I haven't been able to find a similar example in internet.

I am just trying to solve analitically the Sturm-Liouville problem of Schrödinger equation for a particle in a 2D box $[0,1 ] \times [0,1]$. This is what I wrote:

eqn = E*z[x, y] - 0.5 *(D[z[x, y], {x, 2}] + D[z[x, y], {y, 2}]) == 0;
sol = DSolve[eqn, z[x, 0] == 0, z[x, 1] == 0, z[0, y] == 0, z[1, y] == 0, z[x, y], {x, y}]

And this is what I get:

DSolve::dsvar: z[x,1]==0 cannot be used as a variable.

Do you know what can be the problem?