Frustrated by getting a less than helpful "Unknown Error" when I tried to import a Spreadsheet into a Team Site made in SharePoint 2016, I went hunting for a solution.
Trying to Google a solution for hours didn't help, nor did reading several articles on Microsoft's Support website give me any luck.
Until I finally realized that I was logged in with different accounts in Office and SharePoint!
So I finally found the solution to this problem.
I just logged out from Excel, and then logged back in again using the same admin account that I use in SharePoint, and it worked!
A simple fix, yet effective!
I hope this will also help you if you strike the same issue.