I'm not completely sure if I've understood what you're looking for. But if you are using Amibroker, and are trying to Backtest a system which has 'Buy' and 'Sell' signals, but not 'Short' and 'Cover' signals, and are receiving an error when you try to test it, then -
1. Go to the Automatic Analysis window (Analysis Menu -> !Automatic Analysis).
2. Click on Settings
3. Under the 'General' Tab, in the drop down menu next to 'Positions:' select 'Long Only' instead of 'Long and Short' that might be appearing there.
That should allow you to test systems without short and cover signals in them.
Hope this helps.