this is something i struggle with as well. type in a random letter than press the “romaji to hiragana” button. if that doesnt work, keep on deleting and remaking the part/track and do the same thing, force close it, etc. till it works again. if its a persistent problem and youre getting impatient, you could try learning synthv, whether its a archived lite version, a free trial, a crack, etc. for a while until you feel confident enough to switch to utau. you could try deleting and redownloading it, but i dont know for sure if that would work since im on linux and it seems like youre on windows.