Hi,

You can override the action_confirm button and show the warning to the user if the fields are not set.

Sample Code:

# -*- coding: utf-8 -*-
from odoo import api, fields, models, _
from odoo.exceptions import ValidationError


class SaleOrder(models.Model):
    _name = "sale.order"

    def action_confirm(self):
        if not self.fiscal_position_id:
            raise ValidationError(_('Fiscal Position Is Required'))
        res = super(SaleOrder, self).action_confirm()
        return res

Thanks