-You may have pressed Control+spacebar instead of Control+Command when trying to activate or deactivate an insert.-This puts Pro Tools into a " Play Pause " mode that primes the buffers for playback.-To verify this, view your Transport window to see if the Stop button is lit and Play button is flashing.*If this is the case, then Pro Tools behaves as if it is playing back, where you cannot make changes to the DSP allocation (which includes Plug in (de) activation)-To escape this mode, then simply press play and then stop. Pro Tools should return to it's normal "stopped" state.