Business area & not defined (please check your input)

What causes this issue?

A precondition for <GL:account_assignment>assignment</> to the
specified <GL:G/L_account>G/L account</> is the prior definition of a
valid <GL:business_area>business area</>.

How to fix this error?

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.