Sorry that you're getting an error that no preview available for files you're trying to import in Lightroom.
Which version of Lightroom are you using and what is your computer's OS?
The error usually suggests that your do not have enough free space on your primary disk drive (where /TEMP resides). All of those previews need to be opened/created and stored somewhere during import. Lightroom makes extensive use of the /TEMP folder during import and other times. This is why I recommend keeping ~100GB of free space.
Also, it might be possible that you're importing from a Camera that is not yet supported in Lightroom.