Why does my id.me say I’m not eligible

The IRS does state who is eligible under the article See Who is eligible for advance Child Tax Credit payments?. 

To really answer this question it would be important to know if you have the following: "Qualifying children", your address (if you are in the country), your income and other factors.  However, if you are in fact eligible and do not receive payment this will be reconciled on your 2021 taxes. 

You may receive half of the Child Tax Credit for each child in equal payments each month if: 

• For each qualifying child under age 6, you will receive up to $1,800 in advance, generally paid in six monthly payments of $300.
• For each qualifying child age 6 through 17, you will receive up to $1,500 in advance, generally paid in six monthly payments of $250.

These advance payment amounts begin to be reduced if your modified AGI exceeds:

• $150,000 if married and filing a joint return or if filing as a qualifying widow or widower;
• $112,500 if filing as head of household; or
• $75,000 if you are a single filer or are married and filing a separate return.