Before using the AWS Firewall Manager you need to fulfill a number of steps.  As previously mentioned you need to be using AWS Organizations. The account must either join an existing AWS Organization or create a new one. Do bear in mind that the AWS Organization must have all features enabled, not just consolidated billing.

Next, you must designate one of your AWS account to be the Firewall Manager administrator account.

Finally, each member account within your AWS Organization that wants to use the features created by the AWS Firewall Manager administrator needs to enable AWS Config within each region that contains resources that need to be protected.

Once you have configured all prerequisite requirements of AWS Firewall Manager, you can then begin to create your groups of rules and policies to enable you to deploy web ACLs across multiple resources in multiple accounts from a single location.