Though there is no exact equivalent to I miss you in Japanese, there are a few things you could try saying instead.
- Sabishii desu = would mean that you are lonely, but if talking about your partner, it could mean that you are lonely because you miss them.
- Anata ga inakute, samishii kimochi ni narimashita = would mean that you are sad because someone isn't there.
- Aitai" 会いたい = Aitai can be translated as "I want to see you" or "I want to meet you" meaning "I miss you"
Just remember, whether you get the phrase correct or not, the feeling in your voice will let them know how much you care!