The Add Fee is a Option only available to Online Contracts. If the School does not have a Contract built then you will not be able to +Add Fee and successfully Save the Contract.

To see if you have a Contract Created navigate to:
  1. Enrollment Management
  2. Enrollment
  3. Contracts > Manage Contract Forms
  4. Look for the Name of the Contract you are Adding in the Individuals Record, if there is no Active Contract there you cannot add the fee to the User.