After you file

Do you have a copy of your original 2018?  Since your AGI was not  filled in on your 1040X amended return, it must mean there were no changes to your AGI.  You will need to  get your AGI from your 2018 original return.

If you used TurboTax last year you can view your 2018 AGI by selecting Documents on the left hand side of your screen.  Select View Documents from: Select 2018 on the drop down menu.

OR

You can obtain a copy of last year's tax transcript online using IRS link:  https://www.irs.gov/individuals/get-transcript

OR

If you have a copy of your 2018 tax return, your AGI is located on line 7.