To allow your clients to self service and order new licenses, you need to assign a catalog to them.
You can select a catalog to be used as the global catalog. This will then be available as the default catalog for ordering for your clients.
If you assign a catalog specifically to a client, then this will override the global catalog and the client will see the catalog assigned to them.
Log into the end user portal with your email address -https://portal.s365l.com
Click on Admin
Under Partner Administration click on "Assignments"
On this page you can see the existing assignments and add new ones
Click on "Create Assignment"
Select the Catalog
Select Company assignment type
Select the company to assign it to
Click Create Assignment
This will now be assigned to the client