No matter what I type, the config I typed won't apply. For example I tried to reduce the zombie's speed by putting "speed": 0.05, didn't work. Tried to give a modded mob the ability to fly by "abilities": ["fly"], didn't work.

Provide full content of your morphs.json file.

"Zombie": {
    "abilities": ["sun_allergy", "hungerless"],
    "health": 20,
 "speed": 0.04,
    "hostile": true,
    "hands": true
},
},
"lycanitesmobs:manticore": {
    "abilities": ["fly"],
    "hostile": true
}

I don't know why speed at zombie is a little bit to the left, it isn't on notepad

Your JSON formatting is broken... you need to fix your JSON. In the documentation it says that.

https://jsonlint.com/

At the moment, I’m on mobile, so I can’t fix it. Try yourself.