Hyde here from Shopify.
Do you receive any error messages when trying to upload?
Could I ask you to double-check the file format (you can use our sample file for reference here) and please check that the file size does not exceed 15MB? Also could you please ensure you are using Google Chrome and that the file you are using is UTF-8 coded?
If that is all ok, please also ensure to clear your cache and cookies to make sure that isn't interfering with anything, and ensure you're on a solid internet connection too, please.
If none of that helps, please visit our help center and log in to your account to create a support request.
All the best, Hyde.