Explanation and Solution:

The error occurs because Odoo restricts certain operations, such as direct field assignment (record.price_subtotal = ...), in Server Actions for security reasons. Instead, you should use one of the following approaches:

Solution 1: Use a Computed Field

Computed fields are the best approach for dynamic calculations. Here's how you can implement it:

1. Add the Computed Field in Your Custom Module: Define a new computed field in your custom module to calculate the subtotal:

   from odoo import models, fields, api
   
   class PurchaseOrderLine(models.Model):
       _inherit = 'purchase.order.line'
   
       x_currency_exchange_rate = fields.Float(string="Currency Exchange Rate")
       x_computed_subtotal = fields.Float(
           string="Computed Subtotal",
           compute="_compute_computed_subtotal",
           store=True
       )
   
       @api.depends('price_unit', 'x_currency_exchange_rate')
       def _compute_computed_subtotal(self):
           for line in self:
               if line.x_currency_exchange_rate:
                   line.x_computed_subtotal = line.price_unit * line.x_currency_exchange_rate
               else:
                   line.x_computed_subtotal = line.price_unit
   

2. Update the View: Add the new field (x_computed_subtotal) to the Purchase Order Line form/tree view so it is visible to users.

Solution 2: Use a Button Action or Python Script

If you need this calculation as a one-time update (e.g., triggered by a button), create a method in a custom module:

1. Define the Button Action in Your Model:

   from odoo import models, fields
   
   class PurchaseOrderLine(models.Model):
       _inherit = 'purchase.order.line'
   
       def update_subtotal(self):
           for line in self:
               if line.x_currency_exchange_rate:
                   line.price_subtotal = line.price_unit * line.x_currency_exchange_rate
               else:
                   line.price_subtotal = line.price_unit
   

2. Add the Button to the View: Add a button to trigger the update:

   <record id="view_purchase_order_line_form_inherit" model="ir.ui.view">
       <field name="name">purchase.order.line.form.inherit</field>
       <field name="model">purchase.order.line</field>
       <field name="inherit_id" ref="purchase.view_order_line_form"/>
       <field name="arch" type="xml">
           <button name="action_view_stock_moves" position="after">
               <button name="update_subtotal"
                       string="Update Subtotal"
                       type="object"
                       class="btn-primary"/>
           </button>
       </field>
   </record>
   

Solution 3: Use a Server Action (with Context/Write Workaround)

If you must use a Server Action, avoid direct field assignment. Use write() instead:

1. Modify the Server Action Code: Replace direct assignment with the following:

   if record.x_studio_monetary_field_atjMK:
       new_subtotal = record.price_unit * record.x_studio_monetary_field_atjMK
   else:
       new_subtotal = record.price_unit
   record.write({'price_subtotal': new_subtotal})
   

2. Test the Action: Ensure this action works as expected on the Purchase Order Lines.

Preferred Approach: Computed Field

The computed field approach is recommended because it keeps calculations dynamic and avoids manual intervention. It also respects Odoo's ORM structure and ensures maintainability.

In Solution 3, when an automatic operation is created on the purchase order model and the Python code is registered, this does not happen in the final result (when I create a new invoice and manually fill in the currency conversion rate field created by the studio, multiply it by the unit price and then does not show in total)