Encoding 720p 29.97 fps video, options in Adobe Media Encoder are 23.976, 24 or 59.94 fps
The Blu-ray standard doesn't allow this framerate with this resolution.
https://forums.adobe.com/thread/622722

Your options are either upscale your resolution to 1080i/p or to change your framerate to 24fps.
The former will probably produces fewer artifacts.