See this TurboTax FAQ on how to find your AGI
https://ttlc.intuit.com/replies/3301618
You can get free 2018 tax transcript from the IRS
https://www.irs.gov/individuals/get-transcript
If you filed late last year there is a good chance that the 2018 AGI never was entered into the e-file database. Try using 0 (zero) for the AGI.
If you filed an amended return use the AGI on the first filed tax return.
If it continues to reject then the AGI in the IRS database might be in error. Mail filing is the only option.