Shorting Not Supported

This message indicates that your broker does not let you open a short position in the asset class you are trading. Brokers rarely answer this the same way for everything they sell: tastytrade shorts stocks, options and futures but states that "only net long positions may be established" for cryptocurrency, and Alpaca documents that "cryptocurrencies are not shortable" while shorting stocks and options normally.

A signal like this one is rejected before it reaches your broker when the strategy trades crypto and the subscription allows short sides:

{
    "ticker": "BTC/USD",
    "action": "sell",
    "sentiment": "bearish"
}

You have two options. Set Allowed sides on the strategy subscription to long only, so a sell signal closes an open position instead of opening a short one:

{
    "ticker": "BTC/USD",
    "action": "exit"
}

Or trade a product your broker shorts. The broker comparison table on each asset class page shows which products each broker allows shorting for.