The reason for this can be the company setting (csv)  "Department as mandatory on supplier purchase order (Requisition)" is turned on. 

When this setting is activated, the department is mandatory even though this is set as 'not required' on related account (ACC).

If this setting is activated, the user must check that the department (DEP) can be used on the accounts (ACC) that are going to be used in purchase order items (SPO-item).