Assume that the Ukrainian army has broken through. The Russians are on the run. Lacking other good options, Putin uses tactical nukes to halt the advance and bring the situation back under control. It’s his trump card, no?
The problem is that he would be using nuclear weapons to irradiate areas that he now says are legally part of Russia. In other words, the gambit only works if you assume that the annexation is bogus, and that Putin is a liar.
Which he is, of course. Just ask the hundreds of thousands of Russian men fleeing his draft.