If you did not get both of your stimulus payments before filing or with your tax return, then you can amend your return to correct the amount of the stimulus payments you did receive before filing, even if that is zero. Once that section is completed and if your income is not over the limit, then you should be able to claim the stimulus amount you did not receive. It is required to be reconciled on the 2020 tax return.
As you indicated the amended return, Form 1040-X, will be available March 25th. You will use the Federal Review tab to complete the stimulus section after you follow the steps to start your amendment.
First check the stimulus payments the IRS has on file for you.