I did what you told me, I only put that line of code, in the end it gives me this error in the qweb:
object has no attribute 'currency_id'

I forgot to add that this is my own development, I am not inheriting anything.

Below code will take company currency by default.

@api.model
def _default_currency(self):
return self.env.user.company_id.currency_id

currency_id = fields.Many2one('res.currency', string='Currency', default=_default_currency,track_visibility='always')