If you're struggling to clone a class, and receiving an error message, it is likely because there are students within your current class who are already in another class for whichever subject you are trying to clone the class for. For example, it may be that you're trying to clone your maths class as an English class but there is a student who has already been assigned a different English class.
To resolve this:
Hover over Manage and select Students
Select your class from the drop down menu above
From here, you will see a list of students within your class and which maths and English class they are currently assigned to (if you notice any students who are assigned a class they shouldn't be, it could be affecting you cloning the class)
Tick the box next to the names of the students who you need to move out of an incorrect class (you can do multiple at once)
Then from the Action menu above, select Move class > Apply
Using the drop down menu, set them these students to No class and then select Move
Once you've done this, follow the first set of steps above and you should be able to successfully clone the class!