i will make condition end_date auto fill based selection field and start_date field

but have some error like this

File "...\Python\Python35\lib\site-packages\dateutil\relativedelta.py", line 393, in __radd__
    return self.__add__(other)
  File "...\Python\Python35\lib\site-packages\dateutil\relativedelta.py", line 351, in __add__
    raise TypeError("unsupported type for add operation")
TypeError: unsupported type for add operation

my py

from odoo import api, fields, models
from datetime import datetime, date, time
from dateutil.relativedelta import relativedelta
class LicenseID(models.Model):
    _name = 'license.vehicle'
    _inherit = 'ir.attachment'

    name = fields.Selection([
        ('BPKB', 'BPKB'), 
        ('STNK', 'STNK'), 
        ('KIR', 'KIR'), 
        ('UJI_EMISI', 'UJI EMISI'),
        ('KIU/SIPA/KKPKB', 'KIU/SIPA/KKPKB'), 
        ('IBM', 'IBM'), 
        ('STCK', 'STCK'), 
        ('TNKB', 'TNKB')], 
        string='Document Name')
start_date = fields.Date(string='Start Date')
end_date = fields.Date(string='End Date')

@api.onchange('name', 'start_date')
def _change_date(self):
    if self.name == 'STNK':
        self.end_date = self.start_date + relativedelta(years=5)
        self.end_date.strftime('%Y-%m-%d'),

can help me master ??