Hi,

Please go through the Odoo documentation:

https://www.odoo.com/documentation/15.0/developer/reference/backend/orm.html#classical-inheritance

You are creating a new class through Classical inheritance(Prototype inheritance). You can't inherit the existing view in this case(You are supposed to use the new class instead of sale.order). So, create a new view for your new model.

Example

If you check the fields in 'sale.order', you can see these fields:

transaction_ids = fields.Many2many('payment.transaction',
                                       'sale_order_transaction_rel',
                                       'sale_order_id', 'transaction_id',
                                       string='Transactions', copy=False,
                                       readonly=True)
authorized_transaction_ids = fields.Many2many(
        string="Authorized Transactions", comodel_name='payment.transaction',
        compute='_compute_authorized_transaction_ids', readonly=True, copy=False)

The authorized_transaction_ids is a computed field which depends on the transaction_ids field. So, changing the field will cause another error. The tag_ids field is also there.

During development, if you are not intending to update any field, then please use a different name for the new fields to avoid unnecessary errors and to not break existing workflows!

Now, you are supposed to add 3 fields in 'sale.order' and a button in its form view. I have refactored the code here(please make the changes as per your requirement). I think you are adding the customization through the module "custom_addon"(based on the path in the error message.)

custom_addon/models/sale_order.py

from odoo import fields, models

class SaleOrder(models.Model):
    _inherit = 'sale.order'

    field_1 = fields.Char(string="Field One", required=True)
    sale_order_transaction_ids = fields.Many2many(
        'sale.order.transaction', 'sale_order_transaction_sale_order_rel',
        'sale_order_id', 'sale_order_transaction_id',
        string='Sale Order Transactions')
    sale_order_tag_ids = fields.Many2many(
        'sale.order.tag', 'sale_order_tag_sale_order_rel', 'sale_order_id',
        'sale_order_tag_id', string='Sale Order Tags')

    def action_test_function(self):
        print("Test Print")
        return

custom_addon/views/sale_order_views.xml

<?xml version="1.0" encoding="utf-8"?>

<odoo>

    <record id="sale_order_view_form" model="ir.ui.view">

        <field name="name">sale.order.view.form.inherit.custom_addon</field>

        <field name="model">sale.order</field>

        <field name="inherit_id" ref="sale.view_order_form"/>

        <field name="arch" type="xml">

            <xpath expr="/form/sheet/group/group[1]" position="inside">

                <button name="action_test_function" string="Test" type="object"

                        class="btn-primary"/>

            </xpath>

        </field>

    </record>

</odoo>

Ensure that you have added the python files and directories in the respective __init__.py files, added the xml files in the __manifest__.py, and added "sale_management" module in the depends list.

Regards