Look in your Amazon Alexa App ‘device’ list for products that are set to control your TV and disable those that you don’t use and leave just the one to control your TV - Usually the Sonos device has the same name as your Beam’s ‘Room’ name followed by TV - so "Lounge TV" (just as an example) and it also has an icon that looks like a TV - if you see that duplicated by the way, then try disabling/enabling each one until the TV works. You can even try renaming the device to TV.
Hope that helps to sort your issue.
If the on/off command was working before, you might also try unplugging your TV and Beam from power for about a minute and rebooting your router.