If you need to iterate over each record in the recordset and call the super() method for each record individually, you can still do so. However, you should ensure that the super() call is made outside of the loop to avoid calling it multiple times unnecessarily
def action_set_lost(self, **additional_values):

Remember to place the super() method call outside of the loop.

    result = super(CRM, self).action_set_lost(**additional_values)

    for record in self:

        partner_id = record.partner_id

        cursos = record.x_interesado_en_curso

        self._cr.execute("DELETE FROM x_product_product_res_partner_rel_cursos_en_lista_de_espera WHERE res_partner_id = %s AND product_product_id = %s", (partner_id.id, cursos.id))

    return result

Hi,

Please check if you are getting multiple records in both partner_id and cursos by printing them.The reason is probably because you are trying to access id from a record set.

Hope it helps

Thanks for your help, but I'm still experiencing the error. I'll attach it.

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/odoo/addons/base/models/ir_http.py", line 237, in _dispatch
    result = request.dispatch()
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 688, in dispatch
    result = self._call_function(**self.params)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 360, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/odoo/service/model.py", line 94, in wrapper
    return f(dbname, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 349, in checked_call
    result = self.endpoint(*a, **kw)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 917, in __call__
    return self.method(*args, **kw)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 536, in response_wrap
    response = f(*args, **kw)
  File "/usr/lib/python3/dist-packages/odoo/addons/web/controllers/main.py", line 1352, in call_button
    action = self._call_kw(model, method, args, kwargs)
  File "/usr/lib/python3/dist-packages/odoo/addons/web/controllers/main.py", line 1340, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/usr/lib/python3/dist-packages/odoo/api.py", line 464, in call_kw
    result = _call_kw_multi(method, model, args, kwargs)
  File "/usr/lib/python3/dist-packages/odoo/api.py", line 451, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/odoo/addons/crm/wizard/crm_lead_lost.py", line 14, in action_lost_reason_apply
    return leads.action_set_lost(lost_reason=self.lost_reason_id.id)
  File "/mnt/extra-addons/module-resources-odoo/models/crm.py", line 29, in action_set_lost
  File "/usr/lib/python3/dist-packages/odoo/fields.py", line 4061, in __get__
    raise ValueError("Expected singleton: %s" % record)
Exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 644, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 302, in _handle_exception
    raise exception.with_traceback(None) from new_cause
ValueError: Expected singleton: res.partner(33547, 33131, 33062, 32237)

Hi,

The singleton error typically occurs when you are trying to access a recordset that should contain only one record, but it contains multiple records.

Please try as below:

def action_set_lost(self, **additional_values):
    result = super(CRM, record).action_set_lost(**additional_values)
    for record in self:
        partner_id = record.partner_id
        cursos = record.x_interesado_en_curso
        if partner_id and cursos:
            self._cr.execute(
        "DELETE FROM x_product_product_res_partner_rel_cursos_en_lista_de_espera WHERE res_partner_id = %s AND product_product_id = %s",
        (partner_id.id, cursos.id))
    return result

Hope this will help you

Thanks