Video Format Compatibility with GoPro cine studio
GoPro Studio allows constant frame rate H.264 .mp4 and .mov files. Your camera might use a variable bitrate which doesn't seem to be supported by GoPro Studio (Source).

You can transcode the video to a constant frame rate video by using the tool Handbrake and choosing a constant frame rate in the "Video" tab. Choose the "High Profile" preset for a good quality encoding or choose your own quality settings.

You might also want to force a certain framerate that comes close to your average frame rate instead of using "Same as source" if Handbrake makes a automated decision.