Hi,

Please refer to the code below:

from odoo import models, _

from odoo.exceptions import UserError

class StockPicking(models.Model):

    _inherit = 'stock.picking'

    def button_validate(self):

        for picking in self:

            if picking.quality_check_fail:

                raise UserError(_(

                    "You cannot validate this transfer because one or more quality checks have failed."

                ))

        return super(StockPicking, self).button_validate()

Hope it helps.