Clarifications

If you see in the submit server the error "missing_component" the problem is that you are missing a required class. You can also get this error if you are not defining a generic class correctly. For example, your SortedLinkedList class is not being defined as expected. I would recommend that you try submitting your project immediately (if you have not done so yet) so you can identify this problem (your code may work in Eclipse but not in the submit server).
The linked list in our project has a head and tail reference. The next link of the last node points to null.
Methods addToEnd(), getLast(), retrieveLastElement() rely (and must use) the tail reference.
The head and tail must be set to correctly when list operations update the list.
Your code must be efficient. For example, you will lose credit if you perform an unnecessary list traversal(s) during the implementation of a method.
Notice that for the SortedLinkedList class:

1. The add method returns a SortedLinkedList

2. The remove method returns a BasicLinkedList (remember the list must be sorted after the remove). Returning a BasicLinkedList just limits how you can use the remove method, but not what it does. For example, you can do slist.add("Blue").add("Red"), but you may not do slist.remove("Blue").remove("Red"). Doing slist.remove("Blue") by itself is fine.