Hi,

Please try below code 
from odoo import models, fields, api

class ProjectTask(models.Model):

    _inherit = 'project.task'

    @api.model

    def create(self, vals):

        task = super().create(vals)

        # Only create subtasks if this is NOT a subtask already

        if not vals.get('parent_id'):

            predefined_subtasks = [

                {'name': 'Requirement Gathering'},

                {'name': 'Design'},

                {'name': 'Development'},

                {'name': 'Testing'},

                {'name': 'Deployment'},

            ]

            for subtask_data in predefined_subtasks:

                self.create({

                    'name': subtask_data['name'],

                    'project_id': __manifest__.py

'depends': ['project'],

Second Way 

UI-Only (Automated Actions - Odoo Studio)

If you're using Odoo Online (SaaS) or want to do this without code:

Go to: Settings > Technical > Automated Actions

Create a new action:

• Model : Project Task
• Trigger : On Creation
• Action To Do : Create a new record

Add multiple “Create Record” actions with:

• Model: project.task
• Fields:
  • Name : Requirement Gathering
  • Parent Task : Record ID
  • Project : Project ID

i hope it is helpfull

Hi,

Please refer to the code below:

from odoo import models, api

class ProjectTask(models.Model):

    _inherit = 'project.task'

    @api.model

    def create(self, vals):

        task = super(ProjectTask, self).create(vals)

        # Check if project or any condition needed

        if vals.get('project_id'):  # Or any project-specific logic

            subtasks = ['Design', 'Development', 'Testing'] # sample redefined subtask

            for sub in subtasks:

                self.create({

                    'name': sub,

                    'parent_id': task.id,

                    'project_id': task.project_id.id,

                })

        return task

Hope it helps.