Can just mail in if I don't know what my AIG was for2018

The IRS will reject it. If you cannot locate it on your Turbo Tax account you can get a transcript from :

https://irs.gov/individuals/get-transcripts

Yes, you can mail your return.  Here are some additional tips that might help.

You can find your 2018 Adjusted Gross Income (AGI) by:

•  Looking at line 7 on page 2 of your 2018 Form 1040; or
• You can request a transcript of your 2018 tax return from the IRS to ensure you have the right number. You can request your transcript online at the following link: Get Transcript 

Check these items as well to ensure you have the correct number.  If you:

• Filed late last year after mid-November or your return was processed after that time—try entering 0.
• Amended your return last year—make sure you’re using the AGI from your original return, not the amended one. (If you only have a 1040X form, it’s on line 1, column A).
• Rounded your AGI to the nearest dollar—be sure to round up when it’s 50 cents or more and down when it’s 49 cents or less.
• Filed with your spouse last year—you and your spouse have the same 2018 AGI. (Don’t split the AGI amount or enter 0 for one of you).

Unfortunately, without the correct number or a match to the IRS system, your only option would be to print and mail in your return. 

This article will give you instruction on how to print and mail your return.