Your code assumes that all elements within lists will be a list. A list may contain just numbers. So in a case where, a = [ [2, 3], 4, [3] ] and b = [ [3, 5], [5, 6], [4] ], your code will attempt to evaluate the length of the second element in a (which is an integer 4 and has no length).

Your procedure needs a third conditional that checks if only one of the two inputs is a list. Something like :

if is_list(a) != is_list(b):
      return False

I tested your code on several test cases and none of them produced an error but one returned the wrong value.

print same_structure([1], 7) returns True

For reference, these were the test cases I ran:

print same_structure([ [2, 3], 4, [3] ] ,[ [3, 5], [5, 6], [4] ]) == False
print same_structure(3, 7) == True
print same_structure([1, 0, 1], [2, 1, 2]) == True
print same_structure([1, [0], 1], [2, 5, 3]) == False
print same_structure([1, [2, [3, [4, 5]]]], ['a', ['b', ['c', ['d', 'e']]]]) == True
print same_structure([1, [2, [3, [4, 5]]]], ['a', ['b', ['c', ['de']]]]) == False
print same_structure([],[]) == True
print same_structure('','') == True
print same_structure([],['']) == False
print same_structure('',3) == True
print same_structure(3,'audacity') == True
print same_structure([1, [], 1], [2, [5], 3]) == False
print same_structure([1,2,3,[1,[1],2], [[1,2]]], [1,2,3,[1,[],2], [[1,2]]]) == False
print same_structure([1,2,3,[1,1,2], [[1,2]]], [1,2,3,[1,[],2], [[1,2]]]) == False
print same_structure([1], 7) == False
print same_structure([[1,2,3,['d']],4,[1,['g','g',[]]]], [['h',1,'f',[8]],'y',['k',[4,7,8]]]) == False
print same_structure ([1,[1]],[1,[1,2,3]]) == False