When uploading your Subscribers the file needs to be in a .CSV file.
In row one of your file, it must contain the headers exactly spelled out like this example:
As you can see from the above example, when uploading your Subscribers, their are 10 data fields available that you can include. You don't need to use all 10 fields. The only one that is mandatory is Phone.
For example, if you only want to include Phone, First Name, and Last Name.. that is perfectly fine. Just include those three headers in your .CSV file in columns A, B, and C. And the do not include the rest in the headers, remove Email - Tags.
Or if you just want Phone, just put that header in column A, remove First Name - Tags, and then upload your file.
One issue we most commonly see is the file not being properly formatted, due to line endings of each row. Different computer operating systems are using different "invisible characters" when it comes to ending a row in a .CSV file. This is especially common with MAC users.
They way to fix this is to copy the contents of the file, and paste it into a new workbook, and re-save the file as a .CSV file with a new file name, and save it to your desktop. Then try uploading the new document.