You can only update one IDoc at a time

What causes this issue?

The ALE distribution inbox can only process one IDoc at a time. Mass
processing BOM IDocs is not allowed. This applies for material BOM
(BOMMAT message type), document structure (BOMDOC), order BOM (BOMORD),
plant assignment (PALMAT) and material variant (STTMAT) distribution.

System Response

IDoc inbound processing is cancelled. The IDoc remains in the inbox
with status 51 (error during posting).

How to fix this error?

Ensure that the ALE inbox has finished processing one IDoc before the
next one begins.

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.