Problem in Backorder Processing

I am facing a problem in backorder processing

Let me explain the scenario

System shows available quantity as 100 quantity for some ‘x’ material, now I place order for 100 quantity for that material but the system confirms only 70 quantity when check the availability overview in co09 there was an open order for 30 quantity dated 2 years ago when I tried to do the backorder processing thru co06, the system doesn’t allow me to select that particular line it shows the error “please select the line that can be changed.”

I checked that order in va02 in that status for that material is showing completed that has been delivered and billed completely. I don’t know what to do now kindly guide me to solve this issue