I mailed my tax return last year. Is it possible it has not been processed? I am certain I am entering the correct AGI for 2019 but it is rejected.

Yes.  It is very possible.  With the pandemic the IRS is behind in processing all mail.  Try entering 0 for your AGI.

Also, since you did not receive your second stimulus, did you claim it on your 2020 tax return as the Recovery Rebate Credit?

In TurboTax Online, to claim the Recovery Rebate credit please do the following:

1. Sign into your account and continue from where you left off
2. Click on Federal in the left-hand column, then on Federal Review on the top of the screen
3. On the next page titled Let's make sure you got the right stimulus amount, click on Continue
4. Follow the interview
5. TurboTax will determine whether you are entitled to any additional stimulus
6. Any stimulus amount remaining due to you will show as a credit on line 30 of your form 1040.