Your square matrix is singular. Extract from help:
A matrix is singular if its determinant is 0. It is nearly singular if it has a high condition number. These conditions make it difficult to compute the inverted matrix used to solve the problem. The lsolve function may fail or may produce meaningless results in these cases. You may wish to use other decompositions for nearly singular matrices.
Your square matrix is singular. Extract from help:
A matrix is singular if its determinant is 0. It is nearly singular if it has a high condition number. These conditions make it difficult to compute the inverted matrix used to solve the problem. The lsolve function may fail or may produce meaningless results in these cases. You may wish to use other decompositions for nearly singular matrices.