I see on others peoples card they have their rank on it but mines doesn't . Why?

Usercards use an old ranking system, plus they only update once every 24 hours. Don't worry about it.

Usercard rank will show when you'll get brown belt (500 Qi).

On my usercard, my rank always says 0.

that's obviously the problem I see it only showing on some peoples ._. ^

Ikr, That showed 0 for me too once. Just dont worry about it.