Here are some steps to help determine why a file from a Teams conversation could not be downloaded:

1. Check the file size to ensure it is within the size limit of file uploads to Teams.

2. Check the browser version and modify as necessary to a supported version that works with Teams.

3. Check to see that the files, SharePoint, and OneDrive integrations are all working properly.

4. Check that the user has the correct access level to the files they are attempting to download.