I have a pomeraniam and I think it's abnormal. She doesn't like to walk on the leach, I treid many ways such as giving her treats, pet her, ask her favourite person to train her, just no use! She just lying down on the ground and give me that pity look. I tried collar at first, she's OK with it.. but when I attached the string to it, it just stay on the ground.

Try letting her play in the collar and give her lots of praise and treats (Make sure it is not to tight) then attach the leash and act like nothing changed and continue to praise her and play with her. She will learn the idea they the leash is a good thing. I'd advise trying this first in the house were it is more familiar sounds and sights.
If this does not work, attach the leash to her and let her just hang out with it on (Keep a good eye so she doesn't get tangled) just put the leash on and walk away and let her do whatever, it will help her adjust to having a leash on.

How old is she? Does she have a history of people being rough with the leash?

Have you tried a harness instead of a collar? She might be more comfortable with the leash attached at her back instead of her neck. For toy breeds, harnesses can also be safer.

I tried with the collar first, then I attached the string to the collar. But she knew it, and react like before

Thanks. Now, she hate the leash until she bite it. We even try to use a then string as a collar, but she still knew it.. what a sensitive dog..