During the Create Delivery (VL01), SAP prompt you a log that Material xxx is blocked.

During the Sales Order Create/Change for that material,  there might be some update termination that causes the locked of the material  (Windows hanged or power failure).

You can check using transaction SM12 - type an "*" at the User name field.

Check whether the material is in the locked list.

If it is in the locked list, asked the user to log out before you delete it from the locked list.

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SAP Material Management Interview Questions, Certification and Configuration Books

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