File a 2021 return before April 15. Too late for 2020.
The economic impact payments from 2020 are no longer available.
If you were not be claimed as a dependent on a 2021 tax return, go to this IRS website for your account and see if the IRS sent you the EIP3 in 2021 for $1,400 - https://www.irs.gov/payments/online-account-for-individuals