I fixed this problem, when you export your list from any mailing services you will get a csv file. The csv file has alot of columns and squarespace specifically list how to do it in order.
what i did was deleted every single column in the csv file besides the column holding all the emails. Once the emails column is the only column with data (should now show as first column) you'll then proceed to save the file. Now youre all good to upload the csv file. I did this with 40,000 emails i exported from my mailchimp. Hope this helps out.