No we cannot tell you why you are not eligible since we have no access at all to your information.  Did you file a 2020 tax return that claimed a dependent child who will be under the age of 18 at the end of 2020?

Have you read the information about the CTC advance payments on the IRS site?

There will be monthly child tax credit payments going out beginning in July 2021, with some of the CTC still left as a refundable credit on your 2021 tax return next year.

https://ttlc.intuit.com/community/tax-topics/help/how-will-the-stimulus-package-impact-me/01/1393859...

https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021

https://www.irs.gov/newsroom/irs-announces-two-new-online-tools-to-help-families-manage-child-tax-cr...

IRS.gov/childtaxcredit2021, 

https://www.irs.gov/credits-deductions/child-tax-credit-update-portal

https://www.kiplinger.com/taxes/602921/child-tax-credit-update-irs-sending-letters-to-families-who-m...

https://www.kiplinger.com/taxes/602431/child-tax-credit-2021-who-gets-3600-will-i-get-monthly-paymen...