During your file import you received an anomaly "The group was not found. Are you sure of the code?" Verifications to be carried out from your account:
1. Go to the group management page in Settings and click Groups on the left.
2. Check that the group is created.
3. Check that the "group-code" in your file matches the group-code. Feel free to copy and paste it.
3. Make sure there are no spaces in the group-code cell of your file
4. After these checks/corrections you can re-import! In the event of a persistent error, do not hesitate to contact the support team.