Hi @tirsit!
Based on your XLSForm, odkmeta expects your CSV file to have a column named start. However, when the do-file runs and actually tries to import the CSV file, it does not find a column named start. Does your CSV file contain a column named start?
I see that another user encountered a similar issue, also involving start. That user was also using KoBoToolbox. Note that odkmeta does expect the submission data CSV files to match the ODK Briefcase export CSV format. The last time I checked, KoBo's CSV format does not match the Briefcase format, and KoBo also isn't working with Briefcase. However, it's been a while since I checked, so you might want to try exporting from KoBo using Briefcase. If you're able to export using Briefcase, you should be able to run the odkmeta do-file on the resulting CSV files. (If you try exporting from KoBo using Briefcase, please report back either way about whether you were able to export successfully!)
Hope this helps!