Hi Moses Lucele 

This message is a consistency control. You have today stock on this product for this site, so you cannot change the stock management to not managed anymore. If you did a mistake by activating stock management on this product-site record, you can try to issue all the stock linked to this product-site and then you should be able to change the stock management field. 

Wild guess.

I will ASSUME that you were trying to delete this item?   Yes?  No? 

You can't delete items with stock transaction records.   Inactivate the record so it can't be used again.