Hi Sergio,

Because the field ref does not exist in the model project.task. If you want to call a field in the view definition of a model, then that field should be defined in that model. 

Here you want to get the Customer reference which is defined in the res.partner model, ie, Customer. So there is only one field in project.task through which you can reach the Customer, ie, the partner_id field. Through that field you can access the ref field defined in res.partner.

For that, a better option is to define a new related field in project.task, which allows you to access the customer reference and show it in the project.task model.

Inorder to define that new field, you can just create a new custom module, by looking at the Odoo tutorial. And in your module, in the .py file put this:

class project_task(models.Model):
    _inherit = 'project.task'
    ref = fields.Char(related='partner_id.ref', string='Customer Reference')

Then in the .xml file put this:

         <record id="view_task_calendar_inherited" model="ir.ui.view">
            <field name="name">project.task.calendar.inherited</field>
            <field name="model">project.task</field>
            <field name="inherit_id" ref="project.view_task_calendar"/>
            <field name="arch" type="xml">
                <field name="name" position="before">
                	<field name="ref" />
                </field>
            </field>
        </record>

This will bring Customer reference in the calender view of project.task