This can be solved by following these steps:

1. Open the Macintosh HD > Applications > Digidesign > ProTools folder.

2. Inside this Pro Tools folder, single click on the "Pro Tools LE 6..." icon, then click "Get info" from the File menu.

3. In the Pro Tools LE information window, look for the button next to "Open in the Classic environment." If there is an X in the box next to this option, click to remove that X.

4. Close the Pro Tools LE information window and try launching Pro Tools LE.