Sales area & & & is not defined for customer

What causes this issue?

This <GL:sales_area>sales area</> has either not been designed (i.e. no
entry in TVTA), or it refers to another sales area for customers
(customer division/customer distribution channel in detail screen
TVTA).In other words, the customer master data of the sales area
specified corresponds to that of the reference area and only for the
latter is the customer master data and the hierarchy maintenance
allowed.

How to fix this error?

You can maintain common master data in Customizing, using menu path
Organization -> Assignments -> Logistics -> SD.

Error message extract from SAP system. Copyright SAP SE.