what am I doing wrong? (please help in pyhton)

p.s. codes are given below with errors...

DLList.py code:

MaxQueue.py code:

SLLQueue.py:

SLLStack.py code:

DLList Implementation 2(1) Test Case Explanation Passed Remove from empty doubly-linked list - Passed +0.25 Add and get from doubly-linked list - Failed 0 Logs: Add and get from doubly-linked list Traceback (most recent call last): File [codePost Test] Add and get from doubly-linked list: line 59, in output = TestCase () File [codePost Test] Add and get from doubly-linked list: line 39, in TestCase student_list.add(idx, ele) File "/DLList.py", line 69, in add raise IndexError() IndexError Test Case Explanation Passed Points Removing from empty MaxQueue - Passed +0.25 Add and remove from MaxQueue 0 Logs: Add and remove from MaxQueue Traceback (most recent call last): File [codePost Test] Add and remove from MaxQueue: line 137, in output = TestCase () File [codePost Test] Add and remove from MaxQueue: line 30, in TestCase answer_queue.add(integer) File "/MaxQueueCP.py", line 12, in add SLLQueue.add(self, x ) File "/SLLQueue.py", line 22, in add self.tail.next = self.n AttributeError: 'NoneType' object has no attribute 'next' \# pass def remove(self) object: removes and returns the element at the head of the max queue \# todo SLLStack Implementation 1 ( 1 Test Case Explanation Passed Points Remove from empty stack - Passed +0.25 Remove all elements from stack - Failed 0 Logs: Remove all elements from stack Created SLLStack with input: ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k'] Removed elements in this order: k Expected removal order: kj i hf e dcba tack > pop0 > if self. n==0