Unknown account assignment not defined for use here

What causes this issue?

You have entered the <GL:account_assignment_category>account assignment
category</> "unknown account assignment" for the item. However, in the
case of <GL:purchase_order>purchase orders</> and
<GL:scheduling_agreement>scheduling agreements</> an exact
<GL:account_assignment>account assignment</> is necessary.

How to fix this error?

Change the account assignment category to one for which an account
assignment is defined (for example, cost center etc.).

Error message extract from SAP system. Copyright SAP SE.