That is a rather odd error message on your computer, and I'm not sure what to make of that, or why the printer is making an error. Once you have printed the first side, are you taking out all of the paper from the input paper tray before loading the first printed sheets back in? It may be looking for the paper to be removed first and then loaded with the first side printed sheets to know that you've loaded the paper back in for the second side to print, otherwise it may think you haven't done that. There is no separate duplex software you need to install. If you loaded the printer driver from the installation disk, that is all you should need.
Per the manual:
5. Go to the product, and remove any blank paper that is in the tray.
6. Take the printed stack from the output bin, and then insert the stack printed side down into the tray with the top edge feeding into the product first.
7. At the computer, click the Continue button to finish the print job.
Also:
Per the User Manual, these are the possible error conditions that match your statement:
1)
Attention light is blinking, and ready light is off.
The product has experienced an error that can be corrected at the product, such as a jam, an open door, the absence of a print cartridge, or the absence of paper in the proper tray.
Inspect the product, determine the error, and then correct the error. If the error is the absence of paper in the proper tray or the product is in a manual feed mode, put the proper paper in the tray and follow the onscreen instructions, or open and close the print-cartridge door to resume printing.
2)
Attention light is blinking, and ready light is on.
The product has experienced a continual error from which it will recover on its own. If the recovery is successful, the product continues to the processing data state and completes the job.
If the recovery is unsuccessful, the product continues to the continuable error state. Try removing any media from the media path and turning the product power off, and then on again.