My teacher said if you play lined fretless to play exactly on the line is that right ?? If it is boy do I need to improve my left hand intonation
In theory, the string has to be free to vibrate at the fret so the finger has to be right behind it. It's mostly true in practice as well but people's fingers squish differently against the board and you're looking at most fret lines at an angle so some looks as though you're playing behind and others in front. The idea is to learn your bass well by ear using open strings and then look where is your finger when it's in tune. Never assume you're in-tune because your finger is at the right position, especially when you're starting.
The best tip I can impart is to try NOT to make it sound like a fretless.
That's the best advice I can think of. Separate the "normal playing" from "fretless playing". When learning a song, try to eliminate the fretless sound as much as possible.