External id ; This policy maintains a set of best available regions where your subscription can deploy resources. The objective of this policy is to ensure that your subscription has full access to Azure services with optimal performance.

Resource 'xxx' was disallowed by Azure: This policy maintains a set of best available regions where your subscription can deploy resources. The objective of this policy is to ensure that your subscription has full access to Azure services with optimal performance. Should you need additional or different regions, contact support..

There are no options in the allowed deployment region ,in the policy ,matching the country/region for deploying external ID.

deployment of another resource is fine when you can select like us east, south Africa north, but that kind isn't.