Hi,
This error is caused because of multiple values/null values in an instance(Units_ids), thus here we need to loop this field and then we can check the condition; 

Please try this.

@api.onchange('date', 'Units_ids')
def _get_day_of_date(self):
    for r in self:
        if r.date:
            selected = fields.Datetime.from_string(r.date)
            r.date_day = calendar.day_name[selected.weekday()].lower()
            dayoff = r.Units_ids.mapped('dayoff')
            for record in dayoff:
                if record ==calendar.day_name[selected.weekday()].lower():
                    raise UserError(
                        _(F'This Day( {calendar.day_name[selected.weekday()]} ) Is Day Off For This                             Unit , Please Choose Another Unit!'))

Regards

thank you its working and i will study it