Hello @lauy. Which mobile app are you using? Do you have the latest version installed? Are you getting any particular error message or notification when you attempt to save your video?
If you are using Quik for mobile, the suggested steps-to-fix here https://gopro.com/help/articles/question_answer/Quik-for-mobile-Crashes will help. Have a go at them and let us know how it works out. Thanks!