A 2018 tax return can be mailed at any time.
If your 2018 e-filed tax return was rejected for the 2017 AGI, try using a 0 (zero) for the 2017 AGI and re-file. If the return continues to be rejected then you will have to print and mail the 2018 tax return.
See this TurboTax support FAQ for printing a 2018 tax return completed using the online editions - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...
