Getting Error when trying to set Delivery Complete Indicator

My first post here!

I have a 9 line standard PO.  Lines 2-9 all have the delivery complete indicator checked.  When you try to set it for line 1 I get the following error:

"DocHeader

Closure Indicator set at item level 00090. No changes possible."

Any ideas why it won't let me set the Delivery Complete indicator for line 1?

Thank you all!