If social security disability is your only income, you are not required to file a tax return. You are also not required to file a tax return to get the stimulus payment since the IRS is using social security records for paying the stimulus payment to those receiving social security.
The AGI that must be entered to efile a 2019 tax return is the AGI from a 2018 tax return. It is on line 7 of form 1040. If you did not file a 2018 tax return, then you would enter the number 0 for your AGI.