I want to make a class repeating but the system does not offer me any target slots.

This is probably because the Granularity in the Calendar Display Settings is set too high. This value determines when sessions can start, so when this is set for example at Every 30 minutes, this means, the session can start at xx:00h and xx:30h. You should reduce this value if you want to see more target slots:

Go to the app's Settings > Calendar Display
Reduce Granularity to Every 5 minutes, or 10 minutes, experiment with what works best for you.
Set Default Duration to a lower value if you want to squeeze sessions in smaller slots.

Another possible reason why you do not get to see any target slots is that the target slots are already taken in case you are running Base or Pro versions of Bobclass. If you are running Bobclass Studio, then you can schedule sessions in parallel.