Try changing the name of the class to ResConfigSettings and add the get_values and set_values methods to the class, as required.

Here's an example:

https://github.com/odoo/odoo/blob/13.0/addons/hr_expense/models/res_config_settings.py#L6

res.config.settings to add field and save data. Required to set and get value or default method value pass. Based on odoo version will vary. See the below link.

https://www.odoo.com/forum/help-1/question/problems-on-saving-data-to-res-config-settings-in-odoo-12-153308

https://odoo-development.readthedocs.io/en/latest/dev/py/res.config.settings.html

I believe you'd need to add the "action_window" that set's up your module as the context and link's it to your view. Something like this...

     <record id="custom_purchase_module_settings_action" model="ir.actions.act_window">
        <field name="name">Settings</field>
        <field name="type">ir.actions.act_window</field>
        <field name="res_model">res.config.settings</field>
        <field name="view_id" ref="view_res_config_tech_inherit"/>
        <field name="view_mode">form</field>
        <field name="target">inline</field>
        <field name="context">{'module' : 'my_custom_purchase_module'}</field>
    </record>