Is it ok if I put 0 for the 2018 AGI on my taxes even though I don’t know it or have it or have any way of getting it

Yes, you can enter a 0 for the 2018 AGI.

If this is also rejected then you will have to print and mail the tax return.

See this TurboTax support FAQ for printing a tax return for mailing - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...

This is what it looks like