Hi Dalla. You may be having some confusion regarding LOOKBACK? LOOKBACK means the current iteration of the script is still in the lookback phase - i.e., in a lookback bar before active bars where trading can happen.

When you say an Asset is "in lookback", meaning there isn't enough price data yet, you could look at priceClose(<OffsetAsFarBackAsYouNeed>) and if 0 exclude the Asset that way?

HTH.