I added the 2 fields in journal entries 

<xpath expr="//page[@name='aml_tab']/field[@name='line_ids']/tree/field[@name='debit']" position="before">
    <field name="deferred_start_date"/>
    <field name="deferred_end_date"/>
</xpath>

in odoo default the onchange function makes the field set to false 

@api.onchange('deferred_start_date')
def _onchange_deferred_start_date(self):
    if not self._is_compatible_account():
        self.deferred_start_date = False

@api.onchange('deferred_end_date')
def _onchange_deferred_end_date(self):
    if not self._is_compatible_account():
        self.deferred_end_date = False

The other function is this 

def _is_compatible_account(self):
    self.ensure_one()
    return (
        self.move_id.is_purchase_document()
        and
        self.account_id.account_type in ('expense', 'expense_depreciation', 'expense_direct_cost')
    ) or (
        self.move_id.is_sale_document()
        and
        self.account_id.account_type in ('income', 'income_other')
    )

the _is_compatible_account function also called from _get_deferred_tax_key .

and in generate entries function 

def _generate_deferred_entries(self):
    """
    Generates the deferred entries for the invoice.
    """
    self.ensure_one()
    if self.is_entry():
        raise UserError(_("You cannot generate deferred entries for a miscellaneous journal entry."))

What is the specific reason odoo not allowed to generate deferred entries from journal entries . ??

i think one reason is tax . 
odoo only allowed to generate entries from invoice, bills. debit note and credit notes.