Unable to e-file for an extension. I do not owe the IRS, but I still need an extension. Do I have other options?

@famouswhisper17 I see in your other thread that your problem is that you have not yet received your baby's SSN, and you want that so you can claim your child on your tax return.  Critter suggested going on extension if needed in order to wait until October if getting that SSN card takes longer than usual.  If you are getting a tax refund, you do not need an extension---there is no penalty for filing late if you do not owe or if you are getting a refund. You have three years to file a return when you are getting a refund.   If you owe, then you need to pay estimated tax by the July 15 deadline--the extension would just give you more time to file the Form 1040.

What you want to avoid is filing without entering your child as a dependent and then having to amend later, because amending and getting the refund for the amended return takes months.

You say your spouse cannot find his 2018 AGI.  Not sure what finding his AGI has to do with filing an extension--the extension form does not require his AGI.  You need the 2018  AGI to e-file a 2019 tax return.  Between now and whenever you file the 2019 return--you have time to find it. If you filed a joint return for 2018 the AGI is the same for both of you anyway.   If you cannot file by October 15, 2020, your tax return will have to be mailed and by then the AGI will be irrelevant, since you only need it to e-file.

Please read below to get an understanding of how filing separate returns would affect your child-related credits.

If you were legally married at the end of 2019 your filing choices are married filing jointly or married filing separately.

Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $24,400 (+$1300 for each spouse 65 or older)  You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit.

If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return. Some of the special rules for filing separately include: you cannot get earned income credit, education credits, adoption credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. Your limit for SALT (state and local taxes and sales tax) will be only $5000 per spouse. In many cases you will not be able to take the child and dependent care credit. The amount you can contribute to a retirement account will be affected. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. ( Community property states:  AZ, CA, ID, LA, NV, NM, TX, WA, WI)

If  you are using online TurboTax to prepare your returns, you will need to prepare two separate returns and pay twice.

https://ttlc.intuit.com/questions/1894449-married-filing-jointly-vs-married-filing-separately

https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states

https://ttlc.intuit.com/questions/1894449-is-it-better-for-a-married-couple-to-file-jointly-or-separately

You asked about filing separate returns instead of joint---do not do that, or you lose child-related credits. that you would most likely want to receive.