Individuals who have company email which is added to an organization's Azure Active Directory can create their own personal Azure DevOps organizations using the company email. But, Azure DevOps has a feature to restrict creating Azure DevOps organizations using organization's email. As explained in the previous blog (Making Personal Access Token More Secure in Azure DevOps), Add "Azure DevOps administrator" user role to the Azure DevOps admin user to enable the feature.
Go to Azure Active Directory Section of Organizations settings page of Azure DevOps to enable the feature.