Diagnosis
HDMI-CEC (or your TV's version like Simplink, Anynet+) is not properly enabled or configured. This is a handshake issue.
How to Fix It
Enable CEC on your TV. Go into your TV's external device or system settings (often called “HDMI-CEC,” “BRAVIA Sync,” “SimpLink,” etc.) and turn the feature ON. Then, on the soundbar, ensure it's connected via the HDMI 1 (eARC) port.
If That Didn't Work
The specific CEC function may need toggling. In your TV's CEC menu, look for a sub-option like “Device Control” or “System Audio Control” and ensure it's enabled. Reboot both devices after changing settings.
Heads Up
Some TVs have separate settings for “Control via HDMI” and “System Audio Control.” You need both on. If all else fails, use the Bose remote to program its IR blaster to learn your TV's volume codes in the Bose Music app.