See this TurboTax support FAQ for locating a 2015 AGI - https://ttlc.intuit.com/questions/1900840-how-do-i-find-last-year-s-agi
You can also get free transcripts of your federal return from the IRS - http://www.irs.gov/Individuals/Get-Transcript
If the return is being rejected then try using a 0 (zero) for the 2015 AGI. If still rejected then print and mail the tax return.
See this TurboTax support FAQ for printing a return for mailing - https://ttlc.intuit.com/questions/1944348-how-do-i-print-and-mail-my-return-in-turbotax-online