After this, I created an employee promotion, but the Employee Promotion doctype had no letterhead field, so I added a letterhead field to this doctype.
@PRaful_9898 here is link field and in company there is attach field its wrong why are you added custom field in company there is alreay default letter head field in company use that.
Because when I added a letterhead from custom_company_letter_head, a new document is created in the Letter Head doctype with the name of the company, and then this is attached to the default_letter_head_field.