Your screenshot showed up when I posted my reply. Looks like you're in an audio project tab, not a multitrack project tab. This problem should be clip specific.
Close STP and re-open. Then open up that clip in a new audio tab, try the same plug-in. If it's grayed out, I'd certainly look to see whether the clip is mono or stereo and try applying the plug-in to a like format clip. If the plug-in works for a like formatted clip, the clip itself could be the problem.