I didn't get stimulus from 2019. I think they have me down as "can be claimed as a dependent" in error. I'm on SSI. How do I correct this and get stimulus?

I was told to use the non-filers form in error (since SSI recipients don't need to). I'm guessing that is how the "can be claimed as a dependent" box got ticked. But they wouldn't allow for any changes. I was told you can get what is due by filing in 2020, but I have no idea what is needed in this case.