Handling Unit Contains Items that cannot be assigned to Outbound Delivery

Dears All

We are having the following problem:

We create a handling unit in transaction HU02, where we pack the material and the batch in a plant and a storage location.

Later, when we create the outbound delivery, we want to assign the HU created before to that delivery (because in reality those material already have been packed) but the system displays the following message "Handling unit xxxxxxxxxx contains an item that cannot be assigned".

Can Anyone please help with this issue?

Thanks in advance.