When using MacOS effects in Soundtrack Pro, the plug-in may display more channels than you are using.

Programs
Soundtrack Pro 3

Solution

The Apple MacOS plug-ins DynamicsProcessor, MultibandCompressor, and PeakLimiter may show 6 channel meters, even if the audio files in the project have fewer than 6 channels.
The effect will play, render, and export correctly to Multitrack and File Editor files.