Both the parent and the sub customer must be marked as commercial customers with invoicing turned on. See How to Set Up a Commercial Customer.

In Cents Business Manager, go to My Stores > Customers and open the commercial customer.

Click the three-dot menu and choose Commercial Settings.

A Bill with section appears at the bottom of the page.

Mark this customer as the parent (billed for their sub customers' orders), or choose a parent customer to bill this customer's orders to.

Search for the customer, click +, then click Save parent/child customer. To remove a sub customer or change the parent, click the x on its tag.

All linked sub customers show under Bill with.

Go to Invoices in the left menu and click the yellow customers with orders to invoice notification. Sub customers show under their parent in the list.

Click Start invoice to combine all sub customers' orders on one invoice to the parent. (Clicking Invoice on a single sub customer creates a separate invoice billed to that sub customer instead.)

Select the commercial customer and click Next. All the sub customers' orders show under Orders to be invoiced. Click Next.

Add a memo and payment terms, then click Generate Invoice. The invoice goes to the parent. Sub customers don't get a copy.

A parent can have unlimited sub customers. A sub customer can only have one parent.

Sub customers don't receive anything about invoices billed to their parent.

I can't link a customer as a sub customer.
Make sure they're marked commercial and have invoicing turned on.

A sub customer already has a parent.
Remove the current parent first, then add the new one.

The sub customer got the invoice instead of the parent.
You clicked Invoice on that sub customer. Use Start invoice on the parent to bill everything together.