When the user context is within a sub-org (sub organization) using FlexOrgs the "Billing Accounts" AWS report dimension is not available as a "Filters" or "Category" option.
Filters within an Organization do not include Billing Accounts
Filters at the top-level organization do include Billing Accounts
Category selections within an organization does not include Billing Accounts
Category selections at the top-level organization includes Billing Accounts
FlexOrgs
Sub-Org
AWS Cost reporting
This is expected. This filter option is omitted by design in sub-organizations.
You can build a perspective based on the Billing Account name or ID. Use this perspective as a Category or Filter to dimension your report by Billing Account.
Users assigned to the Root Top Level Organization (TLOU) will have this filter available.