Problema com resposta 'chave_invalida' ao realizar pagamento

This response means that the chave that you sent in request isn't from the account that you made the request.

How to find out which one is wrong? "pagador" or "favorecido"

In this case the pagador, favorecido means who will receive the value

body = {
"valor": f"{payment.ticket.price_seller_formatted}.00",
"pagador": {
"chave": self.EFI_FT_PIX_KEY,
},
"favorecido": {
"chave": payment.ticket.seller.efi_key,
},
}

Is it not correct request?
pagador = from
favorecido = to

This is correct, but in this case the credentials that you use are not from the account that own the self.EFI_FT_PIX_KEY.