This error message means that the linear system being solved is singular, which means that there is no unique solution. In particular, the matrix U, which is computed during the solution process, has a zero on its diagonal at the (6,6) position. This implies that the corresponding variable in the system is linearly dependent on the other variables, so there is no unique value that can be assigned to it. This could be due to a variety of reasons, such as having redundant or conflicting constraints in the problem, or numerical issues with the input data.
