Hello Nivetha,
Have you tried the old way?

import base64


pdf = self.env['report'].sudo().get_pdf([invoice.id], 'account.report_invoice')
self.env['ir.attachment'].create({
    'name': 'Cats',
    'type': 'binary',
    'datas': base64.encodestring(pdf),
    'res_model': 'account.invoice',
    'res_id': invoice.id,
    'mimetype': 'application/x-pdf'
})

If you need more explanation go with this tutorial
https://www.cybrosys.com/blog/creating-pdf-attachment-automatically-odoo-13

Or older one:

https://medium.com/hendrasj/how-to-create-attachment-pdf-report-automatically-on-odoo-10-odoo-11-79dd3ad19f23

You will find a way that should work for you also in Odoo 15.


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