I know this discussion is old, but it might still be helpful for someone.
So, are you using the updated excel/csv templates provided along with the offline utility?
If your data is generated in a pre-formatted template by a billing / accounting / ERP system then make sure it is using the updated template compatible with the offline utility version. Call them to resolve the issue or atleast point you to where the issue is and how to fix it.
If you are writing/transferring your transaction data manually to the template, make sure your data is in the specified format and does not include any invalid characters (it usually throws an error when generating JSON, but you never know, there can be exceptions where the utility may overlook an error which will later result in an upload error on the GST portal.
Specifically check for data inconsistency where pre-defined values are to be selected from the drop-down lists available in the template like, Place-of-Supply, Rate, Unit-of-Measure, etc. .... when you select a drop-down cell with a valid data and click the drop-down arrow button, it will open the list which is already scrolled down with the matching entry highlighted in the drop-down list. If that cell contains invalid data then clicking drop-down button will open the list at the very first entry without highlighting anything in the list.
Also check that the dates are entered in correct format and not pasted as a text matter rather than date value.
Hope this helps.