The following customer data pre-requisites and settings need to be maintained correctly on MX in order for your Customers to access the Commerce application and view the products that are available on it:

Sales Area

The Sales Area for the customer contact users (refer to Commerce User Management for additional information) need to be maintained correctly so that the right data can be accessed. This can be done in the 'Sales Data Determination' section which is under 'Accounts' (choose the account that you wish to view) > Contacts. Once you scroll down on that page you will see this.  If the users are not in the right Sales Organization or Distribution Channel they would not be able to view the products or gain access to the Commerce application functionalities. 

Sales information maintained on the account:

Contacts can be assigned to one or more sales area of the account. Only the sales area marked as main on the contact will be taken into account for the order process as well as to retrieve the correct pricing.

Status and Roles of Commerce Users

You will need to ensure that the Status and Roles of the Commerce users are correctly maintained in MX. The roles control whether a user can place an order or if there is a need to send it for review. The status of a user (active/inactive) controls whether or not if the user can log into Commerce or not. You can make these changes by going to Account (choose the account that you want) > Portal Users tab and then making changes to the relevant options as shown in the image below: 

You can also refer to How can I create/provision new Commerce Users? and Commerce User Management to find out more about provisioning users. The mass user maintenance allows the control of roles and status on a set of users rather than by doing it one by one.