Where did you install radiance? If it’s not in the root of the machine (C:\) or in the ladybug_tools folder, then the LBT plugin will not find it.
If you want to run both your legacy honeybee and LBT honeybee using the same version of OpenStudio, see here:
You can also just install an old OpenStudio in the default location, which legacy will use, and install the newer OpenStudio in your ladybug_tools folder.