did the notice ask you to file a return? if so do file but you can't use TT apps. 2015 is no longer supported. you would either need to find other software that will do 2015 and is up-to-date for 2015 or do the return manually.
if you are due a refund, you lost it by not filing timely.
Yes you can complete a 2015 return to refute the IRS bill ... are you sure you did not file it ? Did you get the refund ? Check your records so you can respond to the notice with facts.
Turbotax doesn't support the 2015 tax year any longer ... you can either get the forms directly from the fed/state or seek a local tax professional.