No multilevel structured articles allowed in ME21

What causes this issue?

You want to create/change a purchase order using ME21/ME22 and have
assigned a multilevel structured article (&V1&).

System Response

The system cannot process articles with a two-level bill of material in
transaction ME21/ME22, for example, a display that contains a full
product.

How to fix this error?

Use transaction ME21N/ME22N/MEPO to create purchase orders that contain
multilevel structured articles.

Procedure for System Administrators

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.