CiviCRM not recognizing csv file when uploading
I'd look at the size of the csv, how big is it.
The number of records in the csv, if it is very big I'd break it up.
Look for extra blank lines that should not be there, i.e. clean it up.
Then what kind of csv are you using, open and save again as csv.
I've had these kind of issues, hopefully this helps, good luck!