Why is INJECT saying 'plugin disabled'?

DAWs use dynamic plugin processing and as a result will disable plugins in a channel if there is no audio signal in the channel’s input.

Assigning an audio signal to the channel’s input will stop the DAW from disabling the plugins on the channel.

Alternatively, Pro Tools allows you to disable Dynamic Plugin Processing entirely in the Playback engine, which will prevent plugins from being disabled when there is no audio signal present.