This message appears if you are solving a mixed-integer quadratic programming problem, and the Hessian of the quadratic objective is not strictly positive definite.  When this is true, the QP subproblems solved as part of the Branch & Bound process may not have unique optimal solutions, and hence the Branch & Bound search cannot guarantee an exhaustive evaluation of all possible integer solutions.  If no integer feasible solution has been found by the time the search is terminated, this message appears; otherwise the following message appears.