What may be reason truncate button is not visible on Custom Object in Salesforce?

You will see your profile details (quite a long page with lots of information). We need “Field-Level Security” area (because of our element-type “Custom Field”). Now find the object name your new custom field belongs to and click on the link “[ View ]”. You will be redirected to another page with all custom fields of the custom object where you can cange the visibility of custom field.  If the custom field is not marked as visible, click on the “Edit” button on the top of the page, select the checkbox and save you setting (click on the “Save” button).)