You need to walk through the "File" process to get back to the screen where you can correct your 2017 AGI.  

Go to Step 3 and proceed through the questions and be sure to select the option "I want to E-file".  Continue through the screens and you will be given the opportunity to input your 2017 AGI.

It’s not allowing me to go back .How do I get back to the screen to add my agi?

https://ttlc.intuit.com/questions/2551840-where-do-i-correct-my-2017-agi-in-turbotax-online