Turbotax is rejecting our AGI and is saying it is wrong. Asking for my husbands AGI, but we file jointly, I have no way of knowing what my husbands AGI would be?

On a Joint return there is only 1 AGI.  Enter the same AGI for each of you.  

Try entering 0 for the AGI. The IRS might have processed your return later so the AGI didn't get entered in time.  Especially if you mailed it.  That works for most people. See,
https://ttlc.intuit.com/community/rejections/help/what-if-i-entered-the-correct-agi-and-i-m-still-getting-an-e-file-reject/00/27031

How to correct the AGI in the Online version
https://ttlc.intuit.com/community/agi/help/where-do-i-correct-my-agi-in-turbotax-online/00/26311

If you can’t get it to efile you will have to print and mail it.
https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online/00/26258

Be sure to attach copies of your W2s and any 1099s that have withholding on them.  You have to mail federal and state in separate envelopes because they go to different places.  Get a tracking number from the post office when you mail them for proof of filing.

The IRS uses your prior year AGI to combat fraud.   

You can find your 2023 Adjusted Gross Income (AGI) by:

• Looking at line 11 on page 1 of your 2023 Form 1040
• Requesting a transcript of your 2023 tax return from the IRS. You can request your transcript online at the following link: Get Transcript 

Please click on this TurboTax article for more information. 

Check these items as well to ensure you have the correct number.  If you: 

• Filed late last year after mid-November or your return was processed after that time—try entering 0. 
• Amended your return last year—make sure you’re using the AGI from your original return, not the amended one. (If you only have a 1040X form, it’s on line 1, column A). 
• Rounded your AGI to the nearest dollar—be sure to round up when it’s 50 cents or more and down when it’s 49 cents or less. 
• Filed with your spouse last year—you and your spouse have the same 2023 AGI. (Don’t split the AGI amount or enter  0  for one of you). 

Time-saving Tip: If what you entered is correct and you continue to get a reject, you won't be able to e-file. There's probably a data mismatch at the government, or some other situation beyond your control that prevents your return from being e-filed. In this case, your only option would be to print and mail in your return. 

This article will give you instruction on how to print and mail your return. 

if you filed jointly in 2023, the agi is the same for both of you. line 11 of the 1040. if you filed late use 0 for both. if that does not work, the real problem is at the iRS. You'll have to mail in your returns.

if you amended 2023, the AGI is off the original return not the amended one.