This could be related to your device's support for high-resolution media and its ability to decode such media. There are suggestions at https://community.gopro.com/s/question/0D5Uv00000DU9uRKAT/my-edits-wont-save-to-phone for you to try.
Sorry to hear you're having camera issues. Open the following link on a computer web browser and run through the manual update on your camera - https://gopro.com/update/. Once it's fully updated, you should be able to connect to the GoPro app and use it as normal.