If you already mailed it, it's too late to efile again. So don't. BUT did you try entering 0 for the AGI? Was your 2018 return successfully filed? Here's a couple reasons why it might have rejected.
When did you file 2018? If it was late in the year your AGI wasn't in the IRS database to verify against. So you needed to use zero. And see, https://ttlc.intuit.com/community/rejections/help/what-if-i-entered-the-correct-agi-and-i-m-still-ge...
If your spouse died during 2019 there is a problem efiling and you will have to mail your return
