Hi ,
You can't.. 551 is movement type to post scrap to cost center. You should have qty in block stock to post traxn
If you identify scrap related to Production order , you should post as scrap in Confirmation traxn.. Co11n. Production order variance will be equivelent to Scrap posting when production order in unable to produce full qty. You should TECO order & post variance.
Hope it helps,
Jatin