Error to delete stock movement

how to delete a stock movement whose status is “Completed” here is what the system tells me: To delete the “my product” stock movement, you must cancel it or reset it to the draft state. but I can’t do it

It is recommended to create a new stock movement opposite of the one you want to cancel.
So to move the product back where it came from.
Best to do this using one of the shipments under Stock/Shipments/

I guess the reason is that once a shipment is physically complete, it is impossible to change it, so I guess this is reflected in the tryton modules. Of course it is possible to change as a system administrator, but in that case you already know what you are doing and risk destroying your database.

If you like you may tell about what exactly the problem you are having is, maybe I can help better then.

I installed the stock module to manage a small butcher shop but I realize that there are several sub-stock modules, I am blocked

There is no need to … Just setup a test server which is a copy of your current production system. Make a backup of it, so you can restart quickly when things messed up. Also make sure you cannot operate on your production database when running your test server. You do this by adding a special database user or have the test server on a completely different network.

Then install the base stock module and read the documentation (Stock Module — Tryton module for stock and inventory) and try to understand how it works. Then think what you are missing how the workflow will be etc and look for modules which are extending the base stock. Documentation is at Stock — Tryton Documentation

Using a test server and messing around to get known the system is IMO the best way “Learn by doing”. It also helps to layout for yourself how the whole workflow works when things like expiration dates are added etc. Make it visual and document it so you can then look what functionality is already there and which you have to develop yourself or a service partner.