I have found solution to the problem "Error in Submission". I'm just replying this thread so that it may help someone else facing similar issue.
1) Go to the return for which you are facing the problem. In my case FY 2017-18 Month-December
2) Click on Prepare offline button
3) Click on download json file. It will ask you to wait for 20 mins
4) Download the file and open it in GST offline utility using the second option in the utility which says "Open downloaded retune file from GST portal".
5) Then summary of your return will be displayed. Click on "B2B Invoices - 4A, 4B, 4C, 6B, 6C"
6) Your invoices entered for the month/quarter will be displayed. Now by default "uploaded by taxpayer" click on "uploaded by receiver"/"modified by receiver"
7) Now you will see all the invoices for which the error was generated in your portal. You can find the mistake and rectify easily by either correcting your invoice or asking for your customer to correct it.
PS I already know that this option is also available in GST portal but unfortunately it is not working on the portal as it doesn't show anything.