In the User Management section of Silk Central 16.5, when you are deleting a user, you might get the following error message:

This error indicates that the user you are trying to delete is tied to the template user on your LDAP Server. 

To check this go to Administration -> System Settings, and on click the LDAP Server name and check the 'User Account Creation' settings.

Click the image below for an animated GIF, showing how to navigate to the 'User Account Creation' settings:

If the user you are trying to delete is defined here as your 'template user', you will need to change this user first. Once the user is changed to a different user, you will be able to successfully delete the user.