LINGO has determined that it should be able to solve for a particular variable in a given row. The root finder was not able to converge on a solution. Be sure a solution exists for the row (e.g., the expression: -1=x^.5, would have no real solution). If possible, rewrite the equation in the form x = f(.), where the variable x appears only once and is on the left-hand side of the expression. LINGO is always able to solve expressions in this form, as long as the function is defined and evaluates to a real number. (For example, x = @LOG(-3) evaluates to an imaginary number; and x/0 is undefined.) If this is not possible, then you might try solving the expression by some other method, and enter the variable’s value as a constant in LINGO.