1. Navigate to Setup > Company > General Preferences
2. In the General Preferences page, check the Preferred Subcustomer Form selected.
3. Select an Entity form where the Contacts tab is set to show.
Check that under Lists tab > Relationships
subtab, Show box next to Contacts is checked.
Note : If Preferred Subcustomer Form is blank, check the Preferred Entity form for Customer/Lead/Prospect in the Custom Entry Forms page (Setup > Customization > Entry Forms).