Sales area &1 &2 &3 is not defined for customers

What causes this issue?

The <GL:sales_area>sales area</> specified has either not been defined
(i.e. there is no entry in table TVTA) or it refers to another sales
area for customers (customer division/customer distribution channel in
the TVTA detail screen). This means that the customer master data of
the sales area entered correspond to the data in the reference sales
area and customer master data only needs to be maintained for this
reference sales area.

How to fix this error?

You can display the relevant and therefore valid sales areas for
customers by selecting "Extras" -> "Sales areas" -> "All areas".

Error message extract from SAP system. Copyright SAP SE.