Have you entered all the data correctly?
If you have entered all the data correctly, then Go to Settings >> Reactions and double click on the added chemical reaction
Select the drop-down against velocity units and select kmol/[m3.s] and then click OK.
Close the settings tab and click on solve flowsheet. You will get conversion of nitrogen as 19.79% and hydrogen as 59.37%.
This minor deviation in the conversion value is due to bug fixes in the PFR code. However, the difference is not much.
Please put the DWSIM file in your google drive and share the link here. Will look into it and get back to you.