Please check the credentials and the first domain controller provided in Domain Configuration Settings(check the Right top Corner for domain settings) of ADManager Plus , Login with the same credentials into the domain controller.
Now open the Active Directory Users and Computers , find the User account that you are not able to delete and right click on that user object to check its security settings. Click on the Advanced button and go to the Effective permissions tab. Select the account that is provided for authentication in Domain settings and click on ok. Please check whether the Delete Subtree & Delete option is enabled (Check Security Settings Screenshot) . If it is not enabled Add the required permission by clicking on the permissions tab (Check Adding permissions Screenshot).