How to make the booking, please?

Scenario:

Service A (4 hours) (Resource A) and Class B (Resource B)

Once the class had published (e.g. 10:30 am to 12:00 pm), I can't make a new appointment for Service A at 10 am on the same day.

Any solution to resolve this, please? Different resource had already set to them. 

Much appreciate for assisting. Thank you.