You can only update one bill of material at a time

What causes this issue?

If the error occurred during distribution of
Material BOMs (BOMMAT message type), document structures (BOMDOC) or
order BOMs (BOMORD), please note that the ALE distribution inbox can
only process one bill of material per IDoc.
Plant assignments (PALMAT) or material variants (STTMAT) please note
that only assignments to one BOM per IDoc can be distributed. This does
not restrict the number of assignments per BOM.

System Response

The ALE inbound module cannot process this BOM IDoc. The system can
also no longer accept it if you repost it.
The IDoc remains in the inbox with status 51 (error during posting).

How to fix this error?

Create one IDoc per BOM and per BOM change.

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.