The IRS 2020 e-file database does not recognize the AGI from your mother-in-law's 2019 joint return - it must not have carried over correctly from their 2019 tax return records.
Try entering "0" as her 2019 AGI (this is the default entry for first-time filers, which is probably how the IRS database has classified her for 2020). If this approach does not work, she will likely have to file her return by mail.
For additional information and instructions, see: What if I entered the correct AGI and I’m still getting an e-file reject?
