You can definitely be a soldier if you haven't finished your thesis. Whether you can graduate or not is another matter. If you are not in a hurry, you must finish your thesis and join the army after you finish it. In this case, you can join the army as a graduate, which will be better.

There are two main situations in which college students are allowed to join the army. 1 in the case of not graduating from college, the second case is to join the army immediately after graduation. The two situations enjoy different treatment, but the overlapping time lines of the two situations are the whole university student life, so it is indisputable that you can be a soldier at any time in the university. Of course, if you are about to graduate in your senior year, then I suggest you write your graduation thesis and join the army in the second way.