Can't change interest block at customer invoice

Hi,

I am trying to change the field interest indicator/block for an open customer item. I enter tcode FBL3N, choose customer and click on one of the open items. The form open and I choose change mode. I then choose alternative> additional data and then another form is displayed. Here I have some fields showing, but most of them av greyed out including the field I want to change, which is interest indicator.

Does anyone have an idea where I can make this field possible to change in?

Best regards

Carl-Johan

I will add point directly to an answet that explains this 😊