Hii,

i dont know which version you will use but here is example i hope it is help full for solve your error 
Create/Update Python File: models/sale_order_line.py
from odoo import models, fields, api

class SaleOrderLine(models.Model):

    _inherit = 'sale.order.line'

    net_price = fields.Float(

        string='Net Price',

        compute='_compute_net_price',

        store=True,

        readonly=True

    )

    @api.depends('price_unit', 'discount')

    def _compute_net_price(self):

        for line in self:

            discount = line.discount or 0.0

            line.net_price = line.price_unit * (1 - discount / 100)

Add the Field to the XML View

In views/sale_order_form_inherit.xml

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

    <field name="name">sale.order.form.inherit.net.price</field>

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

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

    <field name="arch" type="xml">

        <xpath expr="//field[@name='order_line']/tree/field[@name='price_unit']" position="after">

            <field name="net_price" readonly="1"/>

        </xpath>

        <xpath expr="//field[@name='order_line']/form/field[@name='price_unit']" position="after">

            <field name="net_price" readonly="1"/>

        </xpath>

    </field>

</record>

in x path set path as per your version 
and if you can use odoo studio then no code needed

in odoo studio

Open Odoo Studio:

• Navigate to Sales > Quotations
• Click the Studio icon (wrench/hammer icon) in the top-right corner

Modify the Quotation Lines:

• Click on a quotation to open the form
• Click into the order lines section
• In the Studio sidebar, click "Add a field"
• Choose "Computed Field"

Configure the Computed Field:

• Set the Label: Net Price
• Set the Type: Float
• Click on "Advanced Properties" or similar
• In the Formula/Expression box, use the formula:

price_unit * (1 - (discount or 0) / 100)

Save and Close:

• Save the field
• Exit Studio

i hope it will work