The fact that you did not indicate on your tax return that you could be claimed by someone else does not mean that no one else claimed you on their tax return. If the IRS records are accurate, some one else apparently claimed you in 2018. Were you eligible to be claimed by someone else such as a parent?
What is your filing status for 2019? Are you required to file a return and if so have you filed it? Did you claim yourself by leaving the box unchecked that someone else could claim you? If you are not required to file a return for 2019, did you use the non filer portal to register for the stimulus?
https://www.irs.gov/coronavirus/non-filers-enter-payment-info-here
