i can inherit res.partner model but i cant inherit sale.order model

i try to inherit sale.order model but it say TypeError: Model 'sale.order' does not exist in registry when i try to inherit sale.order

can someone help me?

from odoo import models, fields, api

class Partner(models.Model):

    _inherit = 'sale.order'

        instructor = fields.Boolean(string="Instruktur")

    session_ids = fields.Many2many('training.sesi', string="Menghadiri Sesi", readonly=True)