When attempting to make a User Inactive an error is received

This issue is likely caused by the User being the default Owner within a few different areas of Salesforce.  From within Setup you can search the following areas in the Quick Find search box:

Web-to-Lead: Ensure the Default Lead Creator is not set to this user.  Click Edit and change to a different user if needed.

Lead Settings: Ensure the Default Lead Owner is not set to this user.  Click Edit and change to a different user if needed.

Support Settings: Ensure that the Default Case Owner and Automated Case User are not set to this user. Click Edit and change both of these to a different user if needed.

Process Automation Settings: Ensure the Default Workflow User is not set to this user. Click Edit and change to a different user if needed

All Sites - Workspaces: Ensure the Default Owner of the Record Ownership in the Preferences tab is not set to this user. Select a different user if needed and click Save

Once the User has been un-associated with the above configurations attempt to mark the User as Inactive again.

See Also:  Unable to Deactivate a System Administrator