This is a small amount of information so we are not able to determine the cause of the error.
Some options:
Hi Devicharan,
Whenever you are getting such errors please check console or logs for what is the exact error type and caused by information, With that you can get some idea to fix the issue, as chris mentioned do the checks, i also suggest some checks, file should be xls format, keys should not be empty if you are going with sync, decimal or boolean value should not be null or empty, template should be selected correctly.
Hope it helps!
Thanks & Regards,
Manikandan K