Ecamm Live's Echo Cancellation feature is a great way to reduce echo and background noise. There are two primary reason why the Echo Cancellation feature may be unavailable (with its checkbox disabled.)
Blackmagic Inputs
Echo Cancellation is not available when using some Blackmagic audio inputs. The checkbox will indicate (Disabled For Current Input).
Built-In Speakers Not In Use
Echo Cancellation is only offered when the Mac's built-in speakers are being used for audio output. When another audio device is used, the checkbox will indicate (Disabled For Current Output). If you still wish to use Echo Cancellation, for example if the connected speakers are not headphone, check the checkbox in the Audio preferences Enable Echo Cancellation For External Speakers.