When editing my sourcing event templates, I can see the Enable multi-round bidding rule, however, I'm not able to change it. How is it possible?

This rule is only available for guided sourcing events, so you cannot enable it for your legacy/classic event templates.

If you'd like to know more about multi-round bidding, please refer to How to work with multi-round events in guided sourcing?

Guided Sourcing > Guided Sourcing Event
Guided Sourcing > Guided Sourcing Event > Guided Sourcing Event Rules