Customers - Customer Rights
Go to "Settings" and choose the sub-section Customers. Here you will find the "Customer Rights" option.
Under "Customer Rights" you have an option to tick "Reject Ordering Without ERP ID". If this tick-box is selected, Customers, that do not have ERP ID defined in the platform, will not be able to order subscriptions.
Customer without ERP ID, trying to purchase a subscription(s), shall have a pop-up window with information to contact their Account Manager, in order to fill in the ERP ID field: