You do not amend a return that was rejected. You only amend a return that has been fully processed. If your 2015 return was rejected that means it was never filed at all, so you do not "amend" it. You just need to complete it and then print, sign and mail it. You are too late to receive a refund for a 2015 tax return. You had three years from the tax filing deadline in April 2016 to receive a refund for a 2015 return. If you owe tax due, the IRS still wants you to pay, though. The IRS will calculate your late filing penalty and interest and bill you later.
To finish a prior year return started in TT online:https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turb
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS and/or your state received the return.