On what TO is based your Layout ? I don't get why you only have merged fields instead of fields from the TO.
Then it appears you don't have any customers. ;-)
Config is not linked to any of the other table occurrences, yet you put merge fields that refer to Config fields in the top left corner. Given your data model, if you use a layout based on Config, this section would show data but none of the other fields would or if you base it on some other table occurrence, then the fields from Config will be empty.
Thus, you need to either refer to a different table occurrence or link config to the other occurrences in a relationship.
Config is just global fields of WHO we are. That prints.
The only TO are represented in the graph.
Invoices are related to Customers by the CustID and Invoice Detail is related to the Invoice by the DocNumber. There are no other relationships. From this Customer information should be available? Or am I mistaken?
Under Billing, you inserted merged fields, ie <<Customers::Company>>, instead of using the field picker to add a field from the TO. Why ?
whether you used merge fields or not isn't the issue. It may not be needed, but it won't by itself keep data from displaying.
If you can't see data from customers on this layout and it's based on one of the SOP TO's shown either you do not have a matching record in invoices that links your layout's current record to a record in customers or the invoices record does not link to any customer in your customers table.
So you need to trace the records back and check match field values as you go.