Hi Try this:

from odoo import models, fields, api

class SaleOrder(models.Model):

    _inherit = 'sale.order'

    payment_day = fields.Date(string='Fecha de Pago', compute='_compute_payment_day', store=True)

    @api.depends('payment_ids.payment_date')

    def _compute_payment_day(self):

        for order in self:

            if order.payment_ids:

                order.payment_day = order.payment_ids[0].payment_date

            else:

                order.payment_day = False

In "_compute_payment_day" method, we check if there are related payment records for each sale order ("payment_ids"). If there are any, we set the "payment_day" field to the date of the first payment record.

Hope it helps