When you delete a Microsoft account from your Microsoft Family, the family membership is removed immediately, but the Microsoft account itself isn't always available to be recreated right away. If you're trying to add a new child or family member using the same email address, Microsoft may still recognize that email as an existing Microsoft account.
If you intended to delete only the family member and not permanently close the Microsoft account, try signing in to that email address at account.microsoft.com. If you can still sign in, simply add the account back to your Microsoft Family instead of creating a new one.
If you permanently closed the Microsoft account, there is usually a waiting period before the account is fully deleted. During that time, the email address remains associated with the account and can't be used to create a new Microsoft account. You'll need to wait until the deletion process is complete, or, if you're still within the recovery period, sign in and reopen the account.
Can you clarify whether you:
removed the account from your Microsoft Family, or
permanently closed/deleted the Microsoft account itself?
That will determine the next steps.