Error 'You cannot deactivate a user who is a portal administrator' when you deactivate a User

Note: Customer Portal and Partner Relationship Management (PRM) are legacy Salesforce features. In newer orgs, these may be replaced by Experience Cloud (formerly Community Cloud). The steps below apply to orgs still using these legacy portal features."; Additional Resources: "Error when you deactivate a user who is the default case owner", "Error: You cannot deactivate a User who is receiving cases or notifications", "Error: This user has insufficient permissions to be a portal administrator"

This happens if the user being deactivated is an administrator in 'Partner Relationship Management' settings. Note: This is only available in Salesforce Classic.

Change User from administrator to Portal User

1. Go to Setup.
2. Click Customize.
3. Look for Customer Portal.
4. Go to Customer Portal Settings.
5. Click available portals. The User should be present in the 'Administrator' field.
6. Click Edit and make the field blank.

If "Customer Portal Settings" is not available in Set Up:

1. Go to Setup | Customize | Partner | Setting.
2. Click available portals. The User should be present in the 'Administrator' field.
3. Click Edit and change the administrator.

See also:
Error when you deactivate a user who is the default case owner   
Error 'You cannot deactivate a User who is receiving cases or notifications as part of your Case Assignment or Escalation rules'     
Error "This user has insufficient permissions to be a portal administrator"