Its probably "QuantiyReserved", basically if you raise a Sales Order, add an item onto the line , it "reserves" the stock so no one else can pick it.  If the program crashes at that point it gets stuck.  A couple of ways to clear this would be to run balance ledgers/verify data for Stock.  This can also find/fix other issues.  Alternatively if you're using Sage200 extra or professional, you can ask your business partner to clear it.

also another way to check, depending on the version you are on, is on the stock list headers, right click and amongst the extra columns you can add is one called QuantityReserved, and that will highlight if this is the problem

Hi - will check - the QuantityReserved. However I ran the balance ledger but it hasnt cleared the probalem

If its not that, there will be some form of discrepancy in the stock or traceable tables meaning it cant be selected.  You'll need to get your business partner to investigate.  It'll probably be a problem in one of the traceable tables that is causing it.