fix NameError in #create_or_update_student

lib/edusign/client.rb

@@ -185,7 +185,7 @@ def update_student(student_uid:, first_name:, last_name:, email:, group_uids: []
     def create_or_update_student(first_name:, last_name:, email:, student_uid: nil, group_uids: [])
       @student = student_by_uid(student_uid: student_uid) if student_uid.present?
       @student = student_by_email(email: email) if @student.nil?
-      raise Response::Error, "Student doesn't exist" if student.nil?
+      raise Response::Error, "Student doesn't exist" if @student.nil?
       raise Response::Error, "Student was deleted from edusign" if @student[:HIDDEN] == 1
 
       update_student(student_uid: @student[:ID], first_name: first_name, last_name: last_name, email: email, group_uids: group_uids)