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It sounds like your account may still have a family-related restriction, even if you've already removed the child account. Sometimes these changes take a little time to sync across Microsoft's services.
Here are a few things you can check:
Make sure your account is no longer part of any Microsoft Family group by visiting the Family settings page.
Compare the country/region set on your Microsoft account with the region configured on your Xbox. If one is set to India and the other to United States, it can sometimes cause inconsistencies.
Sign out of your Microsoft account on your devices, then sign back in after a while to allow any account changes to sync.
If you're still seeing the message that a child account is linked, even though it isn't, this is most likely an account-side issue that only Microsoft Support can investigate. They can check for any remaining family associations or restrictions that aren't visible from your account.
When you contact support, include screenshots of the error message and your current account region settings. That will help them diagnose the issue more quickly.