A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Thank you for reaching out. It's possible that the issue you're experiencing is related to the authentication method being used to access the SharePoint site. If you're not signed in to the desktop Word app using the same account that has access to the SharePoint site, you may not be able to save changes back to the site.
To resolve this issue, you can try signing in to the desktop Word app using the same account that has access to the SharePoint site. You can also try opening the file directly from the SharePoint site in the desktop Word app, rather than downloading it and opening it from your local computer.
If the issue persists, you may need to contact your client's IT department or SharePoint administrator for further assistance.