The videos and images can be edited only if they are located in your Photos/All Photos or Photos/Videos folders.
Please make sure that the video you would like to add is located in your iOS Photos app or Android OS Gallery.
If it is not there, e.g. when it is located at some other application's folder, it is likely that the video would not be visible for Clips.
As a workaround, you can send your video to Gallery via AirDrop (for Mac owners), via messenger, an email or any other method of your choice.
Please do not forget checking the compatible formats list.