This message appears when the Polymorphic Spreadsheet Interpreter in Analytic Solver is being used (Solve With = PSI Interpreter), and the Interpreter encounters an error when computing derivatives via automatic differentiation.  The most common cause of this message is a non-smooth function in your objective or constraints, for which the derivative is undefined.  But in general, automatic differentiation is somewhat more strict than finite differencing:  As a simple example, =SQRT(A1) evaluated at A1=0 will yield this error message when the Solver is using automatic differentiation (since the deri­vative of the SQRT function is algebraically undefined at zero), but it won’t yield an error when Solve With = Excel Interpreter and the Solver is using finite differencing.

If you receive this message, create a Structure Report to pinpoint the exact cell formulas that are non-smooth, and confirm that the “Nonsmooth Model Transformation” option on the Task Pane Platform tab is set to “Automatic” to eliminate the non-smooth functions automatically.  If you can’t modify your formulas to eliminate the non-smooth functions, your options are to (i) use a Solver engine, such as the Evolutionary Solver or the OptQuest Solver, that doesn’t require derivatives, or (ii) set Solve With = Excel Interpreter and solve the problem using finite differencing.