The error means that the system has identified cells in the file, in the Email Address column, that are empty. The system is detecting that the cells contain formatting but it is not an email address nor is it visible to the user. This error does not indicate that the file is wholly bad or the import as a whole failed, there are simply empty cells contained in it.
If you want to avoid this error you can do one of the following:
1. Locate the last cell that contains data in the worksheet, and then reset that last cell by clearing all of the formatting that may be applied in empty rows or columns between the data and the last cell.
2. You can copy the rows & columns that actually contain data and paste it into a new Excel worksheet. Save the new worksheet as a .CSV file type, and import it.