DIAGNOSIS: That's a handshake issue between your TV and soundbar, usually because the soundbar is trying to take over as the main audio source before the TV is fully booted. The eARC/ARC system can be a little finicky on startup.
FIX: First, let's do a power cycle to reset the communication. 1) Turn off both the TV and the soundbar. 2) Unplug both from the wall outlet for a full two minutes. 3) While they're unplugged, press and hold the physical power button on the TV for about 30 seconds to drain any residual charge. 4) Plug the TV back in, turn it on, and wait for the Home screen to fully load. 5) Now plug in and turn on your soundbar. This sequence lets the TV establish itself as the boss before the soundbar checks in.
IF THAT DIDN'T WORK: Go into your TV's settings. Navigate to Settings > Channels & Inputs > External inputs > HDMI signal format. Select the HDMI 3 port (your eARC port) and make sure it's set to Enhanced format (Dolby Vision), not Standard. Sometimes it defaults wrong and can't read the incoming signal properly.
HEADS UP: If you're using a really old or non-compliant HDMI cable, it might not support the eARC protocol. You need a High-Speed HDMI cable, preferably one labeled 'Ultra High Speed'. A bad cable is the culprit more often than you'd think.