I am paying for my studies and I have chosen my own theme for my MS thesis. It is in artificial general intelligence and symbolic methods, rather rare theme. I am promising to do all the work fully myself, with some guidance by external researchers. Noone in the department is willing to be the advisor for my theme. I consider this research direction to be of utmost importance, in any case it is far more superior than the machine translation themes to some minor language - themes that are proposed by the department. I am not asking the would-be advisor to be expert in the field, I am asking just formal signature on the papers to continue my own research. Can university refuse to give me advisor? Maybe I can ask my money back? Maybe I ask the university to improve its quality and topicality of its research directions?
See this from the point of the department: They are giving out diplomas saying "we have seen the work produced by this student and certify that it meets these requirements". If there are no specialists in a given topic, how can they certify that you meet those requirements? When you choose a department for doing your masters then it is up to you to determine that they are doing the things you are interested in. So yes they can and should refuse if they feel they don't have anyone with the expertise necessary to evaluate your work. You can always ask your money back, but in all likelihood they refuse.
You can always ask the university to cover different topics, but universities cannot cover all topics so choices have to made. Adding a topic typically means hiring one or more additional professors. That is a really big long term investment. They are not going to make that investment because a student (who is almost done anyhow) wants that topic.
Approach an advisor that you already have a relationship with. Explain that you are an excellent student, providing grades as evidence, if that's true. (If it isn't, I wonder whether pursuing this topic can be recommended.) Explain why you are passionate for artificial general intelligence and symbolic methods, why the research [is] of utmost importance, and perhaps mention why it is superior [to] the machine translation themes...proposed by the department (being careful not to criticise the potential advisor's research domain). Explain that you have found external researchers willing to guide you, providing letters of support as evidence, if those researchers will provide them. You must deliver your message succinctly and coherently.
Can university refuse to give me advisor?
The university aren't refusing: You are refusing to find a topic that the university is willing to supervise.
Maybe I can ask my money back?
Sure, but you're seemingly at fault, not the university, and I think it is extremely unlikely that you have any chance of getting your money back (at least, not in the context of your question).
Maybe I ask the university to improve its quality and topicality of its research directions?
Sure, but it won't help you. (Change takes time.)