Your menu item is most probably there already. You could verify its existence in Settings -> Technical --> User Interface -> Menu Items to ensure your data-key in your __manifest__.py-file is correct and your module is correctly installed/updated.

If this is the case, you are most likely missing the permission to actually see (read) from the packing.order model - thus you can't access its action(s) either. Ensure you have either a security.xml or ir.model.access.csv added to the data-key of your __manifest__.py-file as well (the file names are not fixed but reflect best practices).

A security.xml could look like this:

<?xml version="1.0" encoding="UTF-8" ?>
<odoo>
    <record id="access_packing_order" model="ir.model.access">
        <field name="name">access.packing.order</field>
        <field name="model_id" ref="model_packing_order"/>
        <field name="group_id" ref="base.group_user"/>
        <field name="perm_read" eval="1"/>
        <field name="perm_write" eval="1"/>
        <field name="perm_create" eval="1"/>
        <field name="perm_unlink" eval="1"/>
    </record>
</odoo>

Alternatively, your ir.model.access.csv could look like this:

"id","name","model_id:id","group_id:id","perm_read","perm_write","perm_create","perm_unlink"
access_packing_order,access_packing_order,model_packing_order,base.group_user,1,1,1,1

You could do this manually via UI as well, however, with a model created in a custom module, I would not suggest to do so in the beginning: https://www.odoo.com/documentation/18.0/applications/general/users/access_rights.html