Once you have completed assigning classes to your homerooms you will need to run a correlate before you assign the achievement profiles. 

How to run the correlate (this needs to be done at each school) 

Main menu>achievement> tools> correlate> Go  

The task will be sent to the notification bell, once the task is completed you can assign the achievement profiles to classes. 

Click on the following link on how to assign achievement profiles to classes: