Why won't it let you file? You can not efile a prior year return. You have to print and mail it. To do a state return you have to first fill out the federal return exactly like it was filed for it to flow to the state.
Did you do the federal return in the Online version last year? You can try to download the .tax2015 file to open in the program.
Here's how to finish
https://ttlc.intuit.com/replies/4186197If you can't download the 2015 file you can Contact Customer Service and tell them you bought the program and they should unlock your 2015 return.
https://ttlc.intuit.com/replies/3300041