As Tax Expert @DawnC and Tax Champ @xmasbaby0 said, the only way to get stimulus money now is to file tax returns.
Any missing first and second stimulus will be added to the refund on your 2020 tax return. The third stimulus will be added to your 2021 tax return.
You can’t prepare 2020 online and you’ll have to mail it to the IRS. Send your return by certified mail with return receipt so you have proof of filing.
Go to File your 2020 taxes to get the software.
