DIAGNOSIS The remote's TV control settings have gotten out of sync. It's trying to send volume commands via infrared (IR) to the TV, but the IR code set is wrong or disabled.
FIX You need to re-run Equipment Control setup. 1) Hold the Home button on your remote for 10 seconds to open the remote management menu. 2) Select 'Equipment Control'. 3) Choose 'TV'. 4) Select 'Change TV'. Follow the on-screen steps to test power and volume. It will cycle through IR codes until your TV responds. When it mutes/unmutes, select 'Yes' to save that code.
IF THAT DIDN'T WORK Check the physical remote. Make sure nothing is blocking the IR blaster at the top. Also, try fresh batteries—weak batteries can cause IR signals to be too weak. As a last resort, unpair and re-pair the remote via Bluetooth in Settings > Remotes & Bluetooth Devices.
HEADS UP If you have a soundbar or receiver, you might want to set the remote to control that device's volume instead. You can set that up in the same Equipment Control menu under 'Audio Equipment'.