This usually happens if the download was interrupted. Try the following:
Is there any way to download view only videos from sharepoint? : r/sharepoint i--- Download Video From Sharepoint Link
⚠️ If the video plays but there’s no download button, try copying the link and pasting it into a new tab with ?download=1 at the end (example: https://yourtenant.sharepoint.com/.../video.mp4?download=1 ). This usually happens if the download was interrupted
If the download button is disabled, tech-savvy users often use developer tools to fetch the file. Only do this if you have permission to download the content. If the download button is disabled, tech-savvy users
This method is highly effective if the web interface is buggy or if you need to download multiple video files at once.
If you are the file owner or have edit permissions, you can download the video directly from the SharePoint interface.