To use Firewall Manager in a Region that's disabled by default, you must enable the Region for both the management account of your AWS organization and the Firewall Manager default administrator account. For information about Regions that are disabled by default and how to enable them, see Managing AWS Regions in the AWS General Reference.
To enable a disabled Region
For both the Organizations management account and the Firewall Manager default administrator account, follow the guidance at Enabling a Region in the AWS General Reference.