This may be a one time error. If you experience another error saying ‘Cannot perform this operation on a closed dataset’, then please send us your data so we can take a look at it.
If the error reoccurs please send us your data file. From within CostGuard, click on File > Backup Data. Backup the data only and send the zip file to email@example.com and let us know exactly what you were doing when the error occurred.