Why does my income show 0 when i input ssa1099 income as $15,831.20?

Generally Social Security income reported on SSA-1099 is not taxable unless you have additional income reported on your return. 

So, if that is all you have entered, there is no taxable income and you would not be required to file a return.

If you were filing a return for the purpose of the stimulus payment, it is not necessary.  See the following information from the IRS website:

I am not typically required to file a tax return. Can I still receive my payment?

Yes. The IRS will use the information on the Form SSA-1099 or Form RRB-1099 to generate Economic Impact Payments to recipients of benefits reflected in the Form SSA-1099 or Form RRB-1099 who are not required to file a tax return and did not file a return for 2018 or 2019. This includes senior citizens, Social Security recipients and railroad retirees who are not otherwise required to file a tax return.

Since the IRS would not have information regarding any dependents for these people, each person would receive $1,200 per person, without the additional amount for any dependents at this time.

@tangookin