This indicates that the trader does not have enough buying power to place this order. Please note, in most cases if the stock price is under $5, it will require the trader to trade it in full cash (or 50%), so a required Buying Power needs to be included in your margin rate too. And if it is a Market order, in most cases it will require an extra 5% of holding Buying Power in order to guarantee the order can be filled with market price. For example: Market Order, stock price is $2.5, full cash required, and account’s margin rate is 4:1, then when trying to buy 100 shares, the required/withhold Buying Power needs to be more than $1050(2.5*100*4*1.05), otherwise the order will be rejected due to not enough Buying Power.
(Please note: Different brokers might have different Buying Power/Margin requirements, so it is better to contact your broker and determine in advance.)