I am having trouble e-filing my 2020 return because, I think, the AGI number is not matching up with my 2019 AGI number because I think my 2019 return was rejected too?

You don't know if your 2019 got filed or rejected?  When you efile you get back 2 emails.  The first email only confirms the transmission.  The second email says if the IRS (or state) Accepted or Rejected your efile. Just paying the fee doesn't file the return.  After you paid did you continue on to the very end and hit the big orange Transmit Returns Now button?

When you log into your account you should see the status and if it was Accepted or Rejected, Started, Printed, Ready to Mail, etc.  what does it say for 2019?

If 2019 wasn't filed or filed late or mailed then try entering 0 for the AGI to efile 2020.