There may be several reason, why you are not able to choose the videos from your gallery:
👉 You haven't provided the App access to all Photos. You may change your preferences anytime via your device Settings - Lift - Photos.
👉 You are trying to add a video as a separate element. Currently, the videos may be added to the template either as a background or to the media box of the template. For more information on how to add the video to the template, please, visit the following article.