This means that you've run out of daytrading buying power, which is calculated as part of the Day Trade Margin Call (DTMC) Protection. The calculation is as follows:

daytrading_buying_power = 4 * (last_equity - last_maintenance_margin)

All these values can be obtained from your account info.

For example, if your last_maintenance_margin was greater than your last_equity, then you'll have a daytrading_buying_power of zero. Both of these are calculated at end of the last trading day, so closing overnight positions will not add to your daytrading buying power.

Please visit this webpage to see more information on what is maintenance margin.

Alpaca uses the following table to calculate the overnight maintenance margin applied to each security held in an account:

Maintenance margin
Margin & Short Selling - Documentation | Alpaca
Alpaca API lets you build and trade with real-time market data for free.