Hello, you cannot use super(res_partner, self).create(vals) because res_partner is a model name string in Odoo, not a Python class.

In Python (and Odoo), super() must be called using the actual class name (e.g., Partner) or with no arguments if you're using Python 3+.

✅ Correct options are:

# Best modern way
return super().create(vals)

# Also valid
return super(Partner, self).create(vals)

Hii,

here is updated code 
from odoo import models, fields, api

class Partner(models.Model):

    _inherit = 'res.partner'

    instructor = fields.Boolean("Instructor", default=False)

    kundennummer = fields.Char(

        string="Kundennummer",

        required=True,

        copy=False,

        readonly=True,

        index=True,

        default='New'

    )

    session_ids = fields.Many2many(

        'openacademy.session',

        string="Attended Sessions",

        readonly=True

    )

    @api.model

    def create(self, vals):

        if vals.get('kundennummer', 'New') == 'New':

            vals['kundennummer'] = self.env['ir.sequence'].next_by_code('partner.kundennummer') or 'New'

        return super().create(vals)

it is updated python code 
i hope using this code your error is solved