Error in model. Please verify that all cells and constraints are valid.

As above, means that the internal “model” (information about the variable cells, objective, constraints, Solver options, etc.) is not in a valid form. An “empty” or incomplete Solver model, perhaps one with no objective and no constraints other than bounds on the variables, can cause this message to appear. You might also receive this message if you’ve modified the values of certain hidden defined names used by the Solver, either interactively or in a VBA program. To guard against this possibility, you should avoid using any defined names beginning with “solver” in your own application.