How come Biden still sure Putin will not win in Ukraine ?

He is not at all sure. He is just spreading a message of determination. He knows full well that it's all about Russia at this stage. It is Russia who will determine if Russia wins the war, not Ukraine or the West. The West hopes that Russia will run out of troops, money or enthusiasm. There is no guarantee that any of those will happen. If Putin is determined to stay the course then there will have to be a peace deal where Ukraine concedes territory and the West knows that too and will likely demand it sooner or later. The US/the West are not going to go to war with Russia over Ukraine and Putin knows that.

Basically, expect an ending where Russia gains large parts of the east of Ukraine. It's either that or Ukraine will be the next Palestine.