この理由は、ユーザーテリトリーレコードを作成している管理者以外のユーザーが、次のドキュメントで説明されているユーザーテリトリーを作成するために必要な権限セットを持っていないためです。
https://help.salesforce.com/s/articleView?id=sf.fs_sharing.htm&type=5
「すべてのユーザーの参照」と「ユーザーの管理」のシステム権限を含む、「Field Service Admin Permissions」 (Field Service 管理者権限) が必要となります。他のペルソナによるユーザーテリトリーの作成はサポートしていません。
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