The most common reason for this is your data feed doesn’t recognize the format of the symbol name. Do Edit>>Translate Symbols to Current Service
Another reason may be that you have exceeded the daily Maximum Profit or Loss for the chart. If the Daily Max P/L % is either > 100% or < -100%, then increase the limits in S3 Inputs (or T3 or W3 inputs as the case may be.)
If none of this works, try the instructions here to verify Auto Trade functionality: https://www.sierrachart.com/index.php?page=doc/SpreadsheetTradingTestProcedure.php