This query will show if there is a a class recorded within Admin > Census > Spring > Class Details with a class type of nursery and no students recorded with the class type nursery on their profiles.
If you do have a nursery you may need to update their class type. This can be changed in bulk using the Group Updater (Admin > Students > Group Updater - selecting Class Type as the student attribute).
Then select nursery class from the drop down above the table, click check all to select the students and then update students at the bottom to apply the change.
If you do not have a nursery you will need to update the class type listed in Admin > Census > Spring > Class Details. In this area you are looking at the details for classes that show at the top of the page.
To edit this click the little pencil in the actions column at the end of the class you need to edit.
Once you have updated the class details ensure to click the little tick in the action to save: