Before a new addon module can be installed, the module list needs to be updated.
This can be done from the Apps top menu and the Update Apps List option. It updates the module list, adding any modules that may have been added since the last update to the list. Remember that we need the developer mode enabled for this option to be visible.
Make sure your web client session is working with the right database. You can check that at the top-right: the database name is shown in parentheses, right after the username, a way to have the server limited to using only the intended database. Until Odoo 11.0, this was done with the additional option --db-filter=^MYDB$. Since Odoo 11.0, this is not required, as by default -d ...
