Diagnosis
The continuous red LED flash is a protection mode indicator. It's triggered by a short circuit or DC fault, most likely in the speaker wire connections to the subwoofer.
How to Fix It
Unplug the soundbar and subwoofer from power. Disconnect the thin, two-wire speaker cable from the back of the subwoofer and from the soundbar's SUB OUT terminals. Check the bare ends of the wire for any stray strands touching each other or the opposite terminal. Trim and re-strip if needed, reconnect securely, then power on.
If That Didn't Work
The fault could be internal. With everything unplugged, try powering on just the soundbar alone (without the subwoofer cable connected). If it turns on normally (white light), the issue is isolated to the subwoofer or its internal amp.
Heads Up
Never use thick-gauge speaker wire for this connection. The terminals are designed for the supplied thin cable or 18-22 AWG wire. Forcing thicker wire can damage the terminals and cause shorts.