If you are importing a very large Excel sheet it may take some time where the app is spinning, maybe up to 15 minutes. If the upload should fail after that length of time, here's how you can split it up.
The workaround is to divide the large Excel spreadsheet into smaller, more manageable segments. Here's a step-by-step guide to doing this:
Break down your Excel sheet into several workbooks with fewer columns.
Label each new Excel workbook clearly to keep track of the data segments: "Part1", "Part2", etc.,
Once split, upload each workbook sequentially. When uploading ensure the Delete existing turnsheet columns toggle is turned OFF, and the imported columns will be appended to the end of the turnsheet.