This usually occurs if your file contains too many contacts.
Upload 1000 contacts in a single CSV file.
Sometime, even though the file contains less than 1000 contacts, it gives this error. This is because when cells are deleted, CSV keeps blank rows. These count as rows in the CSV file. You need to delete the rows itself and not the cells. Open the CSV file, select the entire row (blank rows) and delete them. The size of the file reduces. You can try uploading now.