Unable to Post invoice by using Blanket PO

Hi All,

My problem is on posting invoice in MIRO. I have used frame work order(blanket PO) where I assigned U and B as Account assignment  and Item Category.

Then ,I posted an invoice  and the system requested for G/L account. However, I can't enter  G/L account  because fields are displayed in Grey mode. Please suggest me to post the invoice.