I had the same issue before, and what worked for me was downloading a free Excel template. I used that exact sheet to copy my data in, without changing any formatting or column names. After that, the import worked fine. Also, make sure your Excel file is saved in the older .xls format, not .xlsx.
I had the same issue and found that saving the Excel file as .csv before importing worked. Just make sure the format matches exactly—no extra columns or weird characters.