Your description suggests that you did not work on the correct exercise system. To verify this, please open the lesson you mention. If you can request a new exercise system, you can use it to do the exercise. If you can start or resume an existing system, please let me know and I will investigate.
I requested 3 exercise systems in one course and all of them were the same (also seems like request of exercise systems is broken as it allows me to request 3 of them for one course and doesn't save the link to previous one.)