You have attempted to export dual values when the solver is in primals only mode. In primals only mode, dual values are not computed and, therefore, can't be exported. To enable dual computations, run the Solver|Options command, click the General Solver Tab, and select the Prices option from the Dual Computations list box. To enable range computations as well as dual computations, select the Prices & Ranges option from the Dual Computations list box.  (Alternatively, in the LINGO command line enter SET DUALCO 1 to  enable dual computations, or enter SET DUALCO 2 to enable both range computations and dual computations.)