If you are unable to get a transcript, it is the 2016 return that has to be upgraded to PLUS and paid for (29.99) to unlock the 2015 return.
I'll tell you below how to upgrade your already-filed 2016 return to PLUS and pay with credit/debit card. That should unlock all prior returns IF they are in the same account. It will NOT unlock the prior return if it is in a different account. If you have 2 accounts, then the workaround is more difficult.
