Hello TITI,

You are right that the Odoo Community version is entirely free and open-source (the source code is available on https://github.com/odoo/odoo). You can find more information on the Licenses for Odoo Community (and enterprise) here : https://www.odoo.com/documentation/18.0/legal/licenses.html#odoo-18-community-edition

I think that the "Accounting" module you saw is the account_accountant​ or accountant​ * module which are part of Odoo Enterprise. It is possible that your install contains all the standard free modules + the "names" of all the Enterprise modules that suggest you to "Upgrade" to Odoo Enterprise, and therefore you might see in the AppStore some modules that you cannot install for free with your Odoo Community installation

It's very likely however that the "Invoicing" app ( account​ ) is enough for your needs, it contains everything you need to create and receive invoices, but doing your fiscal accounting on it will probably not be doable. I invite you to take a look at this screenshot that illustrates in Odoo 18 the different apps related to accounting. The one on the left called "Invoicing" is part of Odoo Community, while the one on the right "Accounting" is part of Odoo enterprise

A full list of what is included with Community and what is only available with Enterprise is at https://www.odoo.com/page/editions

* To know exactly the technical name of the module you are trying to install, you can enable developer mode ( https://www.odoo.com/documentation/18.0/applications/general/developer_mode.html#activation ) and go back to the app you try to install. Its technical name ( probablyaccount_accountant​ in our case) will be displayed in pink

Hope this helps !

Kind regards,