Dear friend
Try dcpromo /forceremoval in "Run" and you will not get any error and your domain controller will be removed
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Windows Server 2003 domain controllers
- By default, Windows Server 2003 domain controllers support forced demotion. Click Start, click Run, and then type the following command: dcpromo /forceremoval
- Click OK.
- At the Welcome to the Active Directory Installation Wizard page, click Next.
- At the Force the Removal of Active Directory page, click Next.
- In Administrator Password, type the password and confirmed password that you want to assign to the Administrator account of the local SAM database, and then click Next.
- In Summary, click Next.
- Perform a metadata cleanup for the demoted domain controller on a surviving domain controller in the forest.