This message appears when Solver could not find any combination of values for the decision variables that allows all of the constraints to be satisfied simultaneously. If you are using the Simplex LP Solving method, and the model is well scaled, Solver has determined for certain that there is no feasible solution.
If you are using the GRG Nonlinear Solving method, this method (which always starts from the initial values of the variables) was unable to find a feasible solution; but there could be a feasible solution far away from these initial values, which Solver might find if you run it with different initial values for the variables.
If you are using the Evolutionary Solving method, the evolutionary algorithm was unable to find a feasible solution; it might succeed in finding one if you run it with different initial values for the variables and/or increase the Precision value in the Solver Options dialog (which reduces the infeasibility penalty, thereby allowing the evolutionary algorithm to explore more “nearly feasible” points).
In any case, you should first look for conflicting constraints, i.e. conditions that cannot be satisfied simultaneously. Most often this is due to choosing the wrong relation (e.g. <= instead of >=) on an otherwise appropriate constraint. The easiest way to do this is to select the Feasibility Report, shown in the Reports list box when this message appears.