1. Check if your operating system and web browser are supported by Microsoft Classroom.
2. Make sure the attachment is in a supported file type. Most files types are supported, including .doc and .pdf.
3. Update to the latest version of your web browser if possible.
4. Clear your web browser's cache and try to re-open the attachment.
5. Make sure that content filtering settings allow downloads of attachments.
6. Try a different web browser.