When a field is chosen for calculation the access of that field gets limited for admin and users. The calculation field cannot be created or edited but can only be viewed as can be seen in the image below. The field "Amount to be paid" is selected for calculation and that's why only view access is available to enable or disable. Create and Edit access for the field is grid out.

Important: After selecting any field for calculation it is important to save the domain settings by clicking on the "Save Domain" button under access control tab, otherwise the access limit on calculation field will not get applied at the user end and the user will have default access to the calculation field.