This can happen if there is an amendment or change to your tax return after you file or if your 2018 return hasn't been fully processed.
You can check to see what the IRS has in their system by looking at your tax return transcript, which is available here: IRS Transcripts.
You can also try a 0 - which will work if your return isn't yet on file. However, be careful of making too many attempts as your return will no longer be eligible to e-File after too many attempts.
If you are unable to match the AGI with the IRS records, which sometimes happens for reasons we can't explain, then you will need to print your return and mail it in for processing.