There is a “gotcha” when importing lists from Excel to SharePoint. After selecting the file to import, a message may appear that says, “The specified file is not a valid spreadsheet or contains no data to import.”
Before spending hours trying to fix a file that’s not broken, all you have to do is add your SharePoint site to Trusted Sites in the security tab of your browser. While the message doesn’t seem to reveal that this is the problem, it will likely fix it immediately. You won’t even need to restart your browser.