If you are unable to select the class details, while entering the Enquiry/Application/Admission form. Follow the below path to map the classes
Method 1: With the Admin Level Access
Settings --> Organization --> Assign Sections --> Select the Location and the Academic year --> Click on the Branch name --> Select the classes --> Save
Method 2: With the Branch Level Access
SIS --> Enrollments --> Student Details --> Upload Student details --> Select the Sections against the Class and Click on Save icon