The primary reason for this error is that you are attempting to import data from one tax year into a different tax year. However, there is another reason that users commonly encounter this issue - they are attempting to import a CSV that the system isn't expecting.
If you are following the instructions and getting the proper file for the exchange you are attempting to add data for, the reason you are likely getting that error is because the file is getting edited before being added to the system. This happens even just by opening and then saving the file. So, in order to avoid this, re-download those CSV files from the exchanges and don't do anything to them before importing into our system. Don't open or save them, just export and then import.
This should resolve the issue for the majority of users encountering this error. If it does not resolve the issue, feel free to reach out to email@example.com and provide the CSV you are attempting to import and we'll help you get it sorted!