Hello and welcome, friend~!! It's nice to meet you. You can call me Iris. ouo Most if not everybody in the fandom does.
It's okay to be a bit confused and frustrated. There could be multiple solutions to your problem ;; Maybe you're using a romaji(a, ka, tsu, mi, no) UST and Teto can only sing with a UST that's in hiragana(あ、か、つ、み、の)?
Or maybe you need to set your locale to Japanese?