5. Solver could not find a feasible solution.

This message appears when the Solver could not find any combination of values for the decision variables which allows all of the constraints to be satisfied simultaneously. If you are using the linear Simplex Solver, and the model is well scaled, the Simplex method has determined for certain that there is no feasible solution. If you are using the nonlinear GRG Solver, the GRG method was unable to find a feasible solution, starting from the initial values of the variables; however it is possible that there is a feasible solution far away from these initial values, which the Solver might find if you run it with different initial values for the variables. In either case, you should first look for conflicting constraints, i.e. conditions which cannot be satisfied simultaneously. Most often this is due to choosing the wrong relation (e.g. <= instead of >=) on an otherwise appropriate constraint.