Fast Entry of Characteristics in Purchase Orders - Grey Field

Dear All, I hope you are doing well, with this same Title of the Question nobody answers the Question. Better to write again.

I have have made the prerequisites settings in Customizing of Purchasing.

In PO Screen Char. Display Field is showing with Value when I select one of the Char PO Line items shows desired Char. but I could not enter values there. (It's Grey ).

Please help me with this, Plus I want the same Char. will automatically enter in MIGO classification.

Thanks in Advance