An XY error is a common type of communication error where the wrong piece of information is requested -- or given -- based on faulty assumptions.
The term originated to describe a common problem in IT support settings, in which a user calls and asks how to do action Y; upon questioning, they actually want result X, and erroneously believe that Y is the necessary step to get X. In general, XY errors are errors when the next step in a process is misidentified, especially when due to uncertain knowledge and lack of clear communication.
In the field of philosophy and logic, this is is a type of non causa pro causa, also known as a questionable cause or false cause fallacy; however, these are umbrella terms that also cover other mistakes such as post hoc ergo propter hoc. In cognitive science it might be referred to as a jumping to conclusion bias, although this is also not quite a perfect fit. Surprisingly, this is a comparatively ignored fallacy/bias, which is probably why the IT support term has come to fill the gap. This is an extremely common error to make when learning new systems, and is therefore common in educational settings.
As you may have noticed, I do not much like the phrase XY problem. It sounds awkward and clumsy, and does not match the way we usually talk about other errors and fallacies. In a sane world we would call this a "procedural error" or "procedural confusion", "misidentified cause" or "cause confusion"... or any number of better options. My personal compromise, "XY error", is not very common, and may not be recognized even by those familiar with this sort of error. Unfortunately, and perhaps due to this lack of a general term, you will probably have to explain this type of error no matter what you call it.