It may occur that UN no is not or no longer applicable for an item.

Being in edit mode in item master MMS001, it looks like it can be blanked here.

However, when checking again after it was blanked, it is still there.

UN no in MMS001 comes from MWS001.

In this case, there is no record for this item anymore.

Workaround: 

Create a record in MWS001 and blank UN no in panel E.

Confirm all panels.

When the record is created, it can be deleted again.

Now the UN no also is blank in MMS001.