You can claim it on your tax return next year for 2020. The Stimulus Check is really based on your 2020.
Go to this IRS website for information (Question 33) - https://www.irs.gov/coronavirus/economic-impact-payment-information-center#calculating
A33. Payment amounts vary based on income, filing status and family size. If you filed a 2019 tax return, the IRS used information from it about you, your spouse, your income, filing status and qualifying children to calculate the amount and issue your Payment. If you haven’t filed your 2019 return or it has not been processed yet, the IRS used the information from your 2018 return to calculate the amount and issue your Payment. The IRS is not able to correct or issue additional payments at this time and will provide further details on IRS.gov on the action people may need to take in the future.
If you did not receive the full amount to which you believe you are entitled, you will be able to claim the additional amount when you file your 2020 tax return. This is particularly important for individuals who may be entitled to the additional $500 per qualifying child payments. We encourage everyone to review our “How do I calculate my EIP” question and answer (See question 25).
Keep the Notice 1444, Your Economic Impact Payment, you will receive regarding your Economic Stimulus Payment with your records. This notice will be mailed to each recipient’s last known address within a few weeks after the Payment is made. When you file your 2020 tax return next year, you can refer to Notice 1444 and claim additional credits on your 2020 tax return if you are eligible for them. The IRS will provide further details on IRS.gov on the action they may need to take.