Hi,

Please refer to the code:

from odoo import models, fields, api

from odoo.exceptions import UserError

class HelpdeskTicket(models.Model):

    _inherit = 'helpdesk.ticket'

    @api.constrains('stage_id')

    def _check_close_by_customer(self):

        for ticket in self:

            if ticket.stage_id.is_close and self.env.user.has_group('base.group_user'):

                # staff/internal user trying to close

                raise UserError("Only the customer can close this ticket.")

Hope it helps.