I'm sorry, I wasn't clear in what is happening. Lightroom is trying to open another instance of PM.
Here's what I'm doing. I ingest with PM and cull my photos down to the ones I want to edit in Lightroom. I then import those photos into Lightroom (apple-e in PM) to tone them. When I export the photos in Lightroom, I have the Post-Processing option to open in another application (PM) after export. Previously, this would open the exported folder in a PM window so I could transmit the photos. Now I get the error message. So now I need to navigate to the export folder in order to open the photos.
Long story short, my workflow is PM>Lightroom>PM. Now something isn't letting me open a new PM window from Lightroom.