Why can't I find the consolidated billing option?

Hello.

Are AWS accounts included in Organizations?
https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/useconsolidatedbilling-procedure.html

You can also check whether consolidated billing is enabled in your Organization settings.
https://docs.aws.amazon.com/organizations/latest/userguide/orgs_manage_org_support-all-features.html

You can follow the directions at this link.

Accounts in Organization are part of the consolidated billing family by default - https://docs.aws.amazon.com/organizations/latest/userguide/orgs_manage_org_support-all-features.html - The Payer account will be responsible for paying the bills for all the accounts in Organization (this is by default, you don't have to enable it separately).

Another question is - did you want your Organization to be in the consolidated billing mode only? Or All Features mode (where you have more control over the linked accounts)? By default, you enable Organization with All Features as this just gives you more options in terms of visibility and control (not just cost, but also security and governance).

You can see if your Organization is in the "all features" mode in the Settings - https://us-east-1.console.aws.amazon.com/organizations/v2/home/settings - it will say something like "Your organization has all features enabled". If you already have all features enabled, there's no way to go back to "consolidated billing only" (and you don't really need it since consolidated billing is included in Organization by default).