SSI is Supplemental Security Income. SSI is not reported on a tax return.
If you are not filing a 2019 tax return and you are not required to file a tax return and you do not receive Social Security benefits, then use this TurboTax website to register for the stimulus - https://turbotax.intuit.com/stimulus-check/
Social Security Retirement/Disability/Survivors benefits are reported on a form SSA-1099
If you receive Social Security benefits, see this US Treasury website - https://home.treasury.gov/news/press-releases/sm967
