You need to feed OBS audio source with some signal (non-zero level). Then the option will become enabled.
Or you can try
Filter for your source, for example:
Introduction This is a simple plugin to control stereo pan of audio source in OBS Studio. You can adjust volumes of left and right channel separately. You can adjust stereo pan of each channel. Use case Using a stereo audio interface; If you...
obsproject.com
or any VST-based (v2) with similar functionality.